When using the system preference "AllowHoldItemTypeSelection", you can specify the item type you want for "next available" holds. Unfortunately, on /cgi-bin/koha/reserve/request.pl there is a "Request specific item type:" field at the top under "Hold details" and another under "Hold next available item". Both of these fields get sent, which means that only the 1st one works. (To reproduce, enable that system preference and try to set the item type using the "Request specific item type:" under "Hold next available item". On the hold screen, "Books" won't appear in the "Details" column. Try again using the top field and this time you'll see under "Details" it says "Next available <itemtype> item".) Also, it looks like this "itemtype" isn't passed to CanBookBeReserved. It just goes straight to AddReserve(). (See bug 40475 for more detail.)