at the moment there are six : demons of the ocean, dead deep (world book day) ,tide of terror, blood captain, black heart and empire of night. There is going to be a seventh book coming out in 2011 called immortal war.
There are three now, the fourth one comes out on April 1st, but there may be more to come... MUAHHAH!
1. Demons of the Ocean
2. Tide of Terror
3. Blood Captain
4. Black Heart (Coming April1st)
There is only one book in the book of Revelations.
If you mean to ask how many books are mentioned in Revelation, the number isn't specific. Revelation 20:12 - And I saw the dead, small and great,...