I just got a book that has S as a notation. At first I thought it would be slap, but Tb is slap in that book. The S is usually an octave of the Tb, but I dont have any idea what it can be.

If Tb is "slap" and S is the octave of Tb, then S probably means you "pop" the octave of the note you have slapped.

If you hadnt said S is usually the octave of the Tb I wouldve thought maybe slide?. What Bass Guitar said sounds like it could be right.

Well, I highly doubt it's pop, because S...well wouldn't they just put P? Besides, I'm pretty sure the music isn't popped.
It's not slide because the / or \ is slide. I cant figure it out.