These 2 just came in the mail today:

The Art and Practice of Silver Printing
by Abney & Robinson
This one is mostly about albumen paper print but also briefly discusses gelatin. It would be useful for someone interested in alternative processes.

Photography in the Studio and in the Field
by E. M. Estabrooke
This book cover a lot more ground. It covers collodion, gelatin, silver printing, early camera equipment including shutters, and early enlargers among other things.