I am working in the British Heart Foundation bookshop on Blackboy Hill and we have recently had an large donation of railway books.
I would not claim there are very many rare or connoisseur volumes - mostly popular works such as Bradford Barton albums. They tend to be more loco-oriented, with not so many line or company histories.
Some might be of specialist interest such as volume 10 of the RCTS History of GWR locomotives.
I have been pricing them mostly at £2.50 with a few going up to four or five pounds.
At the time of writing the two classiest volumes in my opinion, both at £10 are:
Barry Curl - The LSWR at Nine Elms Vol 1 ISBN 10: 0954203577
Karl Zimmermann - The 20th Century Limited ISBN 10: 0760314225