Within the Bartimaeus Trilogy, are the magicians capable of doing supernatural feats without the help of the spirits they summon?

In a preface I guess you could call it. It was in Ring of Solomon and it was stated that magicians had no powers of their own except to call and restrain spirits. But numerous examples are given throughout the books that says otherwise.

In book one, it was said that Jessica Whitwell conjured a void with minimal loss of life when Czech spies set a Marid on her, then it also shows her throwing a fireball at Lovelace when he attempted a coup.

In second book, it gives examples of this as well. It summarizes the two year time gap between the books. It also says that Nathaniel learned how to weave Sensory Webs to stop spying and to conjure rapid Fluxes to carry away hostile magic.

So what exactly is the verdict with this?

Answer

It's been a while since I read the trilogy. I just assumed that whenever a magician appears to 'use a spell' or do any magic at all, s/he is really instructing a subject spirit to perform an action. The magician's power depends on the strength of the spirit they are controlling.

So if you had a Marid, say, at your command you could do truly terrible stuff. Perhaps for minor everyday magic maybe you wouldn't trouble your Marid but just use an Imp, any Imp - you would have enthralled loads.

But if you want an in-depth look at this vexed question, try this wiki.