Board Thread:Skyrim/@comment-115.186.32.86-20130513081825/@comment-6881629-20130513082558

Copy/paste from the wiki page -

When Septimus finds the book, Hermaeus Mora might not kill him and the quest becomes stuck. This can happen if staying close to Septimus, trying to talk to him or reading the book before he finishes speaking. Be sure to grab the book on your way out.
 * Solution: Killing Septimus resumes the quest as usual.