Firstly, their collection of historical textbooks going back to the sixteenth century. Just in case you ever need to know how your characters might treat their horses for colic, or how they would manage them generally. Available to peruse by appointment, if you have a specific query in mind and can get down there to meet up with the librarian (email appointments advised).
Secondly, they have a card index of every registered veterinary surgeon in the UK since records began (so Victorian times or thereabouts): name, date of birth, date of registration, date of death (the card I saw also gave a few details of how the subject -- a WWII Flight Lieutenant died). These can then be cross-referenced against the archives of the Veterinary Record to track down any obituaries. Great if you think you've got a veterinary ancestor that you want to track down.
There are some useful resources on the website too, so go check that out if you have any vague interest in veterinary history. Not to mention that they now have a blog, http://rcvscharitabletrustlibrary.wordpress.com/, which I shall be checking out properly later.