Reply to neil:
My manual starts at 5U 13237 I think, as far as serial # breaks go (edit - that's for the parts manual - I don't think my service manual has a serial # break but don't have it available right now to check)
DP, when I was buying D2 manuals about 6-7 years ago, I found that there wasn't just one, single book to cover the whole machine. What I did end up buying was -
-Form #FE030524-02 "Servicemen's Reference Book, Caterpillar Diesel Engines (4" Bore 4 - Cylinder) for D311 Industrial, D311 Electric Set, D311 Marine, D2 Tractor, No.212 Motor Grader" - this manual covers everything engine related, basically from the bellhousing forward.
-Form #FE030441-01 "Servicemen's Reference Book, Caterpillar D2 Tractor, 3J, 5J, 4U, and 5U Series" - this manual covers the rest of the chassis (tracks, undercarriage, final drives, transmission/rear end, all clutches and brakes). This manual even covers the later build D2's with the longer chassis that can have the main clutch serviced without splitting the tractor.
-Form 12238 "D2 Tractor, 50" Gauge, Parts Catalog, Serial Numbers 5U1 to 5U13236 Inclusive" - parts manual for entire tractor.
Since you also have a Cat factory blade and hydraulic unit on your D2, you'll also need books to cover those items. If you're lucky and you still have all the serial number tags on your machine, you can use them to find the manuals to cover those assemblies. I've personally accumulated books that cover the #2A and #2S Hydraulic Bulldozers in the 5G, 7G, 7F, and 9F serial number ranges, also Operator's, Parts, and Service Manuals that cover the #44 Hydraulic Control Systems in the 4W, 7W, 8W, and 9W serial number ranges, plus a Form #12914-2 "Servicemen's Reference Book, Caterpillar Hydraulic Controls and Equipment" that is a good all-around manual for the #41, #44, and #46 Hydraulic Units. Long story short, you'll likely end up with a whole stack of books to cover everything on your machine by the time you're finished. Of my stack of books, it's about 50%reproduction to 50% original, but I'm not a paper collector so it doesn't bother me if some are repro's. Just need the information they contain.