Here's some books:
From Chuck @ Kick-Start [(616)245-8991 or from other sources you may know. If so, then you're welcome to say so.
1. 1940-47 Big Twin Service Manual ( For Knuckle & Side Valve) The only H-D manual that gives solid lifter pushrod adjustment for any Big Twin. (Forty-year old re-production) copies can be purchased though Keenan Tatro (namesake of Keenan Wynn) (619) 233-6605, while supplies last. The pages fell out of mine after a few years of use (now kept together in a plastic sleeve), but the photographs are factory clear. The paper is old style clay-coated stock, no longer available from printers.
2. 1948-57 (Rigid) Service Manual- A mutt compilation of the Knuckle manual and typewriter written pages, in several font styles. Contains info about the "hydraulic unit pushrods" if you have a '48 Pan. Not for the new re-pop motors, but fills a service manual void for early Panheads.
3. 1959-69 Electra Glide Service Manual (99482-69) Still available from Harley-Davidson. The best since the (OEM) 1940-47 manual. Only covers hydraulic tappet/pushrod adjustment, not solid lifter.
4. Clymer Panheads (ISBN 0-89287-565-8). Excellent. Covers solid pushrod and hydraulic tappet adjustment. Contains alot of tips not in the factory manual. 511 pages.
5. How To Restore Your Harley-Davidson (ISBN 0-87938-934-6) The best service/history manual out there for 45;74 & 80 Side Valve; Knuckle & Panhead. 640 pages. Get a copy.
6. Mechanics & Owners Guide 1941-59 H-D (ISBN 0-9672744-0-0) Covers clutch hub liner replacement, solid lifter pushrod adjustment and hydraulic/solid conversion, tools, and tips. No mousetrap, tranny or anything else that is already fully covered in any other manual. 302 pages, which I wrote, with technical information supplied by Mark Stettler.
1. 1911-39 Spare Parts- 45, Singles JD, Side Valve 74 & 80, early Knuckle
2. 1936-48 Parts Catalog (black, reverse-negative, cover) Fairly worthless, except it isn't cluttered with 45 parts like the '54 Spare Parts book. Gives old three and four digit parts numbers. You'll need a "Cross Index of Original Factory Part Numbers" to go with it if you use it to order parts from a supplier.
3. 1941-54 Spare Parts- (gray cover)- Excellent. Gives five digit part numbers. Covers 45;61;74 and 80" machines. Knuckle & Pan. Crazy index though, you have to search through obscure headings to find the part you need.
4. 1957 (1949-57 OHV; '46-52 WL; '51-57 Servi-Cars). [red cover] Excellent. Easy index.
5. 1958-'68 Parts Catalog (99456-68)- Excellent. Easy Index. Illustrates the current Hydra Glide fork parts used today. THE parts book for todays bobber and restoration enthusiast.
6. Parts Catalog FX Models 1971-84 (99455-83C)- Has the (cast) shifter cam for rachet top transmissions (page 67) Using this cam (34019-73) will change a (transmission top-mounted) jockey shift lever pattern to: forward for first, back to neutral, back for second, third and fourth gear.
Legendary Reading Material
1. Harley-Davidson 1930-1941, Herbert Wagner (ISBN 0-88740-894-X) Schiffer Publishing (610) 593-1777. The true account of H-D factory life before the 2nd World War. History of the Knucklehead. This is a good read.