I'm an adult and I loved these books as did my college age daughter. Admittedly, we have gotten into the habit of snitching my 10YO's books, because the 9-12 YOs seem to be getting more interesting reading material than adults.
I will say, Bartimaeus may be targeted for 9-12YOs, but they are a bit dark for that age group. However, the writing is advanced enough for the 13+ crowd. Fantasy has become so repetitive that I had almost stopped reading it, but Bartimaeus feels original, it's not just one of the same old stories.
Bartimaeus is a witty, smart-mouthed demon with no interest in humans. However, an apprentice magician, Nathaniel, has summoned him correctly so he must serve. Throughout the series Nathaniel grows up and becomes less self-centered. The demon and the boy become tentative friends - except that Nathaniel never quite feels he can trust the demon.
This is a great series, it keeps you on your toes. The characters are well-developed and the reader is often anxious for them. The plots make sense. Each book can stand alone, but they are better together. Excellent series.