I'm having the same problem. This is August 2016, two years later and still no answer... I follow your procedure, but while I see the "new book" option, it is greyed out and I cannot click on it.

Solution: That gray option is actually active. (It's not intuitive and I forget it between infrequent Houzz sessions. Maybe the design needs tweaking, hint.) Anyway: Click in that empty field to the right of the + sign. Then type the name of your new idea book and save the active photo to it.