There are a number of places spread around wikitree:
There is: http://www.wikitree.com/wiki/Genealogy_Resource_Exchange
And some are given here: http://www.wikitree.com/wiki/Space:Kitty%27s_Library
And possibly here: http://www.wikitree.com/wiki/Genealogy_Lookups_and_Tombstone_Photos
There are likely other places I am not aware of.
As to categories and free space profiles:
My suggeestion is to first create a category for 321st Infantry Division which can itself be categorized under "US Army Units" and "US Army in World War I". Your G-fathers profile can be added to the "321st Infanty Division" category as well as "World War I US Army Veterans". On the category page for 321st Infantry Division You can indicate there is a book relevant specifically to that category and indicate your willingness to do look-ups, with a link to your profile, where people can then send you a Private Message.
If you wish to go further, you could then create a free-space profile for the book itself, with a physical description of the book - maybe a photo of the book if the cover or frontspiece is interesting. You could also (if copyright is expired) add transcriptions, or an exceerpt if you wish. The book's forward (if it has one) would be interesting while an index (if it has one) would be useful. This profile could be categorized under 321st Infantry and possibly under other ones as well.
Of course if the book has already been digitized like this one: https://archive.org/stream/historyof321stin00john#page/n5/mode/2up you could just add a link to the digital copy, mention that you are willing to verify information from any omitted pages or poorly scanned pages and save some wear and tear on your precious and delicate book.