Catalogs
x3 on the factory service manual. A factory parts catalog for your year mopar is also an indispensable reference book. Both can easily be found used or in reprint book form/CD - PDF format.
As mentioned, these forums (FABO, Moparts) are an excellent resource for real world experience with our beloved A bodies. Lots of reputable suppliers advertise and some even blog on these forums.
Not critical but a nice book to have on the shelf is the Musclecar Color History of the Dodge Dart & Plymouth Duster. Published in 2000, it covers the years 1960-1974. Nice photos, factory equipment and tech specs although it could easily be a much longer book.
One other book that has proved to be wealth of info for me over the years is the Direct Connection Performance Book. At 1400 pages, published by Chrysler in 1980, it collects all of the racing bulletins/papers written by the Chrysler engineers and designers themselves. Exhaustive reference with extensive info on factory high-performance packages, engine and chassis mods for the street and strip, and tons of factory part numbers and descriptions. The book pops up from time to time on ebay however, some of the specific engine and suspension chapters have also been reprinted in smaller books by Mopar Performace.