I hope this helps anyone confused, as I was, about the whereabouts of the YAHS book collection. There's been a big reorganisation within the West Wing of the Brotherton, and the signage hasn't yet caught up.
First, it's important to note the exact location given in the library catalogue. You can check this from home or within the library.
YAHS rare books, and all pre-1850 publications, are securely housed in Special Collections and will probably need pre-ordering. Please check access arrangements ahead of your visit.
Books flagged up with the YAHS label are in the Brotherton West Wing level 2 stacks (mobile shelving). The shelves themselves haven't yet been fully labelled. YAHS books are stored between the stack Archaeology and Theology collections. Within the YAHS set, books are arranged according to topic, starting 'Archaeology' and ending 'Yorkshire'. The catalogue reference should take you easily to them.
Many of the titles which were at Claremont are to be found in the Brotherton's own Yorkshire collection, which is in the main section of the library (BLM), level 2.
This is the situation as I found it on Friday (9 March). Although I know the Brotherton quite well, I needed to ask! There's an enquiry counter to the right as you enter the library itself, and the staff were very helpful.