I also second the recommendation of Psychohistorical Crisis. It's Big SF like, say, Vinge's "Marooned In Realtime," and very enjoyable. However, I definitely don't recommend the Second Foundation Trilogy by the Killer Bs. It was well written (of course), but not good Foundation. It was inconsistent with the original and-- well, I won't post spoilers, but let's just say it introduced an idea which was a great SF concept, but completely inappropriate for the Foundation Universe.
Which is why I say I felt Psychohistorical Crisis was more respectful and true to Asimov's vision than the trilogy by the Killer B's
It's a shame that the novel didn't get authorized by the estate. But in a way, it's better for it as the author didn't really have any restraints and was able to go in a direction without stepping on any toes, like a vision of the world through someone else's eyes. The introduction of the familiar or fam was brilliant and very Asimov.