I think Ken Stewart's "The Soaring Pilot's Manual" is a great post-solo / early-cross country book. Written in the same style as the first book.
That, and Reichmann's epic "Cross Country Soaring" are worth getting.
Wallington's "Meterology for Glider Pilots" has a few sections on thermals and is a worthwhile addition to any library.
I think it's worth reading more than one book on the same topic, and discuss with instructors/pundits too, as theories change through the ages and sometimes new things disprove the old (and vice versa!)
Good luck