After finishing up Expendable, I was looking on Amazon to see what else James Alan Gardner has written. Perusing the results, I couldn't help but notice that the cover of one of his books looked awfully familiar. I went over to my shelf, and sure enough, I owned it. So, I picked up James Alan Gardner's Vigilant and started reading. After a bit of a delay due to moving and starting a new job and what-not, I continued reading. And then, predictably enough, I finished reading.
But enough about logical progressions; let's talk about the book.
Although I didn't realize it immediately (due to not reading the back of the book, which incidentally has spoilers up to about the 3/4 mark, so I recommend following my practice of avoiding it), Vigilant is actually a semi-sequel to Expendable -- it's set in the same universe, and the protagonist of the earlier book is a significant character (though not the protagonist) in this one. The plots are unrelated, but similar in that both have puzzle elements, ancient mysteries, and fast-paced action.
This book has all the virtues of Gardner's first -- witty dialogue; solid, interesting worldbuilding; a well-paced story; interesting puzzles; and likable characters -- putting him at two for two at writing good, gripping science fiction. That's enough evidence for me: I'm putting Gardner on my to-buy list, and my next order from Amazon will be loaded up with the rest of his oeuvre.