Start with the free: https://shapeokoenthusiasts.gitbook.io/shapeoko-cnc-a-to-z/ 
Make: Getting Started with CNC By Edward Ford
The definitive introduction to working with affordable desktop and benchtop CNCs.
The CNC Cookbook: An Introduction to the Creation and Operation of Computer Controlled Mills, Router Tables, Lathes, and More
This is a really good introduction to CNC. It's well written and understandable. It will help you understand some of the vocabulary and gives a good introduction to CNC.
A tutorial sample is available on the website Feeds and Speeds Tutorial for CNC.
Hobbyist CNC Machining --- Wikibook (nascent)
CNC router evaluation procedures by Edward L. Adams
Setup Reduction Through Effective Workholding by Edward G Hoffman
Low-Cost Micromachining Development and Application for Engineering and Technology Education --- Paper on the development of a micro-machining tool with a 2" x 2" x 2" work area
CNC Programming Handbook by Peter Smid
Japanese gitbook: https://www.gitbook.com/book/fablabsendai/how-to-use-shapeoko/details
G codes for the specification of Pythagorean-hodograph tool paths --- Academic paper discussing how curves, arcs and circles are handled and the consequences of representing them as polygons or polylines.
Curve fitting with arc splines for NC toolpath generation --- Yeung, Millan K; Walton, Desmond J, Computer Aided Design. Vol. 26, no. 11, pp. 845-849. 1994
https://a-e-mag.com/ --- Awards & Engraving industry magazine --- mostly focused on laser engraving, but covers other topics.
http://www.cadhistory.net/ originally published as The Engineering Design Revolution https://www.goodreads.com/book/show/17203585-the-engineering-design-revolution-cad-history
Fundamentals of engineering drawing for technical students https://babel.hathitrust.org/cgi/pt?id=mdp.39015009822332&view=1up&seq=9
makers by Chris Anderson --- discusses “the next manufacturing revolution as home fabrication and talks about desktop CNC machines, 3D printers, &c.”
Fab: The Coming Revolution on Your Desktop--from Personal Computers to Personal Fabrication by Neil Gershenfeld --- examines history of the implications of affordable fabrication, including the origin of pyCAM.
Can Emerging Technologies Make a Difference in Development? By Rachel A. Parker (Editor), Richard P. Appelbaum (Editor)
American Machinists’ Handbook and Dictionary of Shop Terms: A Reference Book of Machine Shop and Drawing Room Data, Methods and Definitions by Fred H. Colvin and Frank A. Stanley Second Edition (last version which has entered the public domain and become freely distributable) https://www.reddit.com/r/CNC/comments/43qbv1/best_edition_of_machinerys_handbook/ --- not the Industrial Press’s Machinery’s Handbook now in its 30th edition, see below.
Machinery’s Handbook 30th edition --- a 2896 page book full of useful information from Trigonometry to Speeds and Feeds and much more. Earlier editions can be picked up used on eBay for reasonable prices.
1st edition reprint: https://www.anvilfire.com/bookrev/ind_pres/index1st.php When measured one copy was 183mm x 127mm x 55mm --- one Kennedy tool chest drawer intended for storage of this text was 5.5 x 7.5 x 3  and the size of the drawer changed as the book's page count increased, making it a useful way to date a chest.
Modern Machine Shop's Handbook for the Metalworking Industries by Woodrow W. Chapman --- often suggested as a suitable up-date for the 21st century shop.
Text-book of advanced machine work --- Written by an M.I.T. professor at the turn of the previous century. Includes many tips on machining.
Setup Reduction Through Effective Workholding by Edward G Hoffman
Online public domain
Book: Elements of Machine Work by Robert Henry Smith: https://archive.org/details/textbookelement00smitgoog
Book: Principles of Machine Work by Robert Henry Smith: https://archive.org/details/textbookofprinci00smitrich
Book: Advanced Machine Work by Robert Henry Smith: https://archive.org/details/textbookadvance00smitgoog 
Turning and Mechanical Manipulation, Vol. I : Materials, Their Choice, Preparation and Various Modes of Working Them by Charles Holtzapffel
Turning and Mechanical Manipulation, Vol. 2: Holtzapffel's Construction, Action & Application of Cutting Tools by Charles Holtzapffel
Turning and Mechanical Manipulation: Abrasive and Other Processes Not Accomplished With Cutting Tools by Charles Holtzapffel (Author), John Jacob Holtzapffel (Author)
A number of U.S. Army manuals on metal-working have been written, and since published by the U.S. Government are in the public domain. One place which they are available at is opensourcemachine.org:
- Band Saw Operations
- Fundamentals of Machine Tools
- Principles of Drafting and Shop Drawing
- Lathe Operations
- Metal Properties, Characteristics, Uses and Codes
- Milling Machine Operations
- Machine Shop Calculation
- Welding Operations
Mechanical Practice by United States. Radio matériel school, Washington, D.C. United States. Bureau of Naval Personnel.
Metal Cutting Theory and Practice (Manufacturing Engineering and Materials Processing) 2nd Edition by David A. Stephenson (Author), John S. Agapiou (Author)
Home Machinist's Handbook by Doug Briney
Metal Lathe for Home Machinists by Harold Hall
Metalworking: Doing it Better and Metalworking Sink or Swim by Tom Lipton
Machine Shop Trade Secrets by James A. Harvey
Jigs and Fixtures: a reference book showing many types of jigs by Colvin and Haas 
Interesting Reddit thread about melting 6061 aluminum cans, dross and other issues: https://www.reddit.com/r/Machinists/comments/3606n6/anyone_know_what_type_of_aluminum_soda_cans_are/cra2a3n
(Please see Wood under Materials below)
(Please also see the section below on CNC Joinery)
- Chip Carving Patterns by Wayne Barton
- Machinist-Materials, Plastics Comparison Table
- Designing Plastic Parts For Assembly by Paul Tres
Similarly, the U.S. Forestry Service has many publications on wood. One especially useful example is, Wood Handbook: Wood as an Engineering Material (Centennial Edition). https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm
UK take on it here:http://www.laver.co.uk/content/timber-pdf-downloads (Timber%20properties.pdf seems to no longer be available)
Understanding Wood: A Craftsman's Guide to Wood Technology by R. Bruce Hoadley 
Laser-Beam Interactions with Materials by Martin von Allman 
digital joints --- booklet describing joints cut using a CNC and their usage.
That is also linked at: Fabrication Techniques & Hardware: Joinery, q.v.
Collection of .pdfs: http://www.derekhugger.com/tools.html
Precision Machine Design illustrated edition by Alexander H. Slocum
http://char.txa.cornell.edu/ Art, Design, and Visual Thinking
"The workmanship of risk has no exclusive prerogative of quality. What it has exclusively is an immensely various range of qualities, without which at its command the art of design becomes arid and impoverished."
"The danger is not that the workmanship of risk will die out altogether but rather that, from want of theory, and thence lack of standards, its possibilities will be neglected and inferior forms of it will be taken for granted and accepted."
The Nature and Art of Workmanship by David Pye
- http://www.adobe.com/products/postscript/pdfs/PLRM.pdf --- the "Red Book" http://paulbourke.net/dataformats/postscript/ 
- http://www-cdf.fnal.gov/offline/PostScript/GREENBK.PDF --- Glenn Reid made this freely available, but his site is off-line
- Mathematical Illustrations: A manual of geometry and PostScript by Bill Casselman
Signs and letter-forms
- Creative Lettering Today by Michael Harvey --- Republication of several classic works, Calligraphy in the Graphic Arts, Creative Lettering and Carving Letters in Stone & Wood with additional new information on digital letterforms. The computer information is specific to the Macintosh program FontStudio (no longer available), with occasional references to Fontographer, but includes mentionings of concepts simply not addressed anywhere else.
- Lettercarving in Wood: A Practical Course by Chris Pye
- Choosing and Using Hand Tools by Andy Rae — woodworking tools
- Virtuoso: The Tool Cabinet and Workbench of Henry O. Studley --- beautiful photographs and wonderful writing about one of the most famous tool cabinets
- Quality is Contagious: John Economaki and Bridge City Tool Works
- The leather work book : containing full instructions for making and ornamenting articles so as to successfully imitate carved oak ; specifically written for the use of amateurs by Rosa Baughan
- Leather Work by Adelaide Mickel