I didn't, really. I knew that of course, Bartimaeus would have to return (that's why it's the Bartimaeus Trilogy, duh), but I got the feeling it would be a new master several years later. I'm not sure why, exactly. Maybe because Bartimaeus implies that Nathaniel wouldn't want Bartimaeus around after the rosemary-tin-can thing ran out (he knows his birth name), and because the book ends with Bartimaeus instead of Nathaniel.

I read the back covers of the first 2 books before actually beginning with the 1st book, so I had so no such doubts.