Hi all,
I believe the "Original TR" books you're refering to are the series by Bill Piggott. I agree, they are excellent. Within the past few years, his older TR2 through 6 book (last printing 1991-ish) was revised into two different books with expanded info on the TR2/3 in one, TR4/4A/250/5/6 in the other. He also published "Original TR7 & 8" recently. For good prices, look on Amazon.com. Or check eBay (but be careful, prices vary wildly!).
I also highly recommend the new "How to Restore" series from Roger Williams, along with a separate series of "How to Improve" books. The latter is for those who want to go Vintage Racing or perhaps just want to Hot Rod their TRs. The former is more aimed at authenticity and compliments the Piggott books well, in fact he wrote the foreward in some of them. The "Restore" books are more model specific: one for 2/3, another for 4/4A, a third for 250/5/6 and a fourth covering TR7 & 8. Roger has also written extensively about MGBs. One of his "Improve" books covers TR2 through 4A. Another covers TR250/5/6.
Rogers's books are heavily illustrated and very up to date with good British parts source info and a smattering of U.S. parts sources.
For anyone with dreams of doing laps, the Kas Kastner tuning manuals are now compiled into one book, I noticed TRF has it, possibly others do as well.