A few days ago, I was in mourning for my copy of High Wizardry, by Diane Duane, which is in storage somewhere. Yesterday, what should I see in the library but High Wizardry? Clearly, it was A Sign, so I checked it out and did a fast re-read yesterday and today.
This is definitely my favorite of the series, and probably its high point too (no pun intended). Nita’s kid sister Dairine (and if anyone can tell me how you pronounce that, I’ll be very grateful) has been offered wizardry at a very young age. The younger the wizard, the more vulnerable—but also the more powerful. The Powers clearly want Dairine to tackle a very big problem . . .
I love the last hundred pages of this—sense-of-wonder at its finest—so much that I can forget, between readings, the blatant scientific error behind one of the cool plot devices.  However, reconciling the end of this with the subsequent books is something of a dubious proposition. After this re-read, I suspect more strongly than ever that this was meant to be the last book in a trilogy (as I speculated a few days ago, in a post about A Wizard Alone). To me, this isn’t a book-destroying kind of problem, but it is a minor annoyance to have to pretend that subsequent books take place in a slightly alternate universe.
 If you’ve read the book and are curious, I posted about this to Usenet a while ago. Warning: these naturally contain spoilers. Here’s my original post, and some follow-ups that didn’t thread properly.