"The Ultimate Do-It-Yourself Guide to Rebuilding and Restoring" classic tractors in general, or Farmalls or John Deeres in particular.  I have the Farmall book, and it's a very good reference with more than 200 pages and lots of photographs (mostly black and white) organized into chapters on buying a tractor (and evaluating it before you buy it), troubleshooting, engine repair, clutch and transmission, hydraulics, electrical systems, and more.  It focuses on the original Farmalls, the Letter Series, and the Hundred Series tractors, and provides a good general guide to lots of repairs that can be accomplished with relatively simple tools and patience.  For more complex tasks like servicing the power takeoff unit or torque amplifier, the author suggests consulting a service manual or having the work done by a qualified mechanic. 

A good series of books by Spencer Yost, founder and operator of the Antique Tractor Internet Service.  How to Keep Your Classic Tractor Alive is a 2009 reprint (updated with color photographs) of the author's Antique Tractor Bible, an A to Z primer on the old-tractor business.  It features more than 200 pages covering everything from how to buy and transport an old tractor to how to use, maintain, repair, and paint it, plus chapters on setting up a shop, working with metal, tractor shows and pulls, and more.  It's well illustrated with photographs, many of them close-ups of details relating to the text.  How to Rebuild and Restore Farm Tractor Engines covers some fundamental concepts then provides chapters on engine removal, disassembly, the cylinder head, valve train, engine block, and so on, all the way through assembly and testing.  Spencer follows the actual rebuilding of engines from a Farmall F-20, Ford 8N, John Deere B, and a few others, with clear text and lots of photos to make it an informative read for anyone with a wrench and a willing attitude.  How to Restore Your John Deere Two-Cylinder Tractor continues the series, with lots of photos and detailed captions illustrating restoration of the engine and various components of the two-cylinder John Deeres.

General descriptions of how various tractor components work, and a summary of the restoration process on a Farmall A and a Ford 9N.  No substitute for a shop manual, parts book, and dirty fingernails, but useful for less-experienced mechanics like me in determining what to try themselves and what to hire out.  Out of print now, and superseded in quality by Tharran Gaines' How to Restore Classic Farm Tractors, above.

Excellent, step-by-step descriptions and photos of how to repair a huge variety of magnetos.  Also includes a good general description of basic electrical systems.  The late Mr. Yerigan's expertise was matched by his helpfulness, and his book is a good reference for anyone who didn't have the opportunity to ask questions of him in person.  The original 1994 version has long been out of print, but the book was republished in 2011.