If a book is not showing in the initial list of books available to be assigned, it is probably not marked as being "In Use". To show all the books in the system in the Web App, tap/click the Change Handbook List buttonChange handbook liston the menu bar, then select Show all books, including obsolete and not "In Use".  Within the Windows App, there are radio buttons near the top of the Assign Handbooks dialog with a similar option.

Tip: You can use the Windows App to set the book as In Use so that it shows up in the default list. Go to Admin > Curriculum, double click the book, then check the "In Use" checkbox for any translation used.