Reply to drujinin:
Bob LaVoie's D2/R2 Photo Archive is a good book for the info you seek!
Great thread!
As others have mentioned, if you can find one, the Barnard book is a very good reference book. It doesn't go into a ton of detail, but enough for most questions that might come up. It has good illustrations from spec sheets and brochures to complement the info too.
Orlemann's Caterpillar Chronicle is also near the top of my list. There are two other Orlemann books that contain much of the same info. Classic Caterpillar Crawlers contains much of the same text as Chronicle, but only covers up to 1955 and only tractors. Caterpillar Century seemed to me to be a refresh of much of the same info as Chronicle, but with some different, larger pictures. All three are similar in that they weave the details into more of a story format rather than just a collection of specs and descriptive paragraphs. Part of this is the details are complemented with anecdotes passed on from some of the people involved. Some of those recollections are difficult to prove, or in a few cases, don't quite align with other facts, but they still make for a good story.
For detailed reference on early Holt and Best history, it's hard to beat Caterpillar's Roots by Jack Alexander. It maybe doesn't "flow" as well as some of the other books, but there is no lacking for detail and he does a good job of citing where the information came from if you want to do more research on your own. Roots stops at the Holt/Best merger though.
The Caterpillar Pocket Guide also comes in handy if you just need quick specs on a Cat tractor model from 1925-1955.
You also can't forget about the ACMOC newsletter. There have been some very good articles in there over the years. The one Lorry Dunning did on the 2-Ton is the best info on that model I have ever come across. Unfortunately, I don't think there is any index to those type of articles.
Dave