Ha! Yeah, I think the sixth one just recently came out.
Fifth, actually. If anyone's interested, the books in order are:
Leviathan Awakes
Caliban's War
Abaddon's Gate
Cibola Burn
Nemesis Games
And I just finished Nemesis Games. Awesome stuff.
They could easily break each book down into an entire TV season and still never run out of ideas. The books almost read that way already, with recurring "guest star" characters and callbacks to previous novels that you only get if you're really paying attention.
"You can't take the Razorback! We are gone and gone!"