Trapped in Time
Spoilers. There are often spoilers here. I sometimes forget to point that out.
The last of the DS9 YA novels, Trapped in Time is almost as much of a departure from the original format as the previous book, Honor Bound (see below). Okay, it's a Nog and Jake adventure, but instead of being set during the first couple of seasons, it's set during the Dominion War, with Nog at Starfleet Academy. Jake's visiting Earth with Miles O'Brien, and the three set out to France to visit a scientist who's researching time travel. Before you can say "look out he's a changeling and he's going back through time to change history," well. Off they go to World War II, just before the D-Day Invasion of Normandy.
Miles, Jake, and Nog have to find the changeling, who's taken the form of a Nazi officer and is heading to Paris to inform the regional Nazi military command that the Allies are going to hit the beaches in just a few days. They have run-ins with Nazis and French resistance fighters and Jake gets a bit of a crush on a French girl, at one point saving her life. It feels a little too familiar -- it seems like everyone who does time travel will meet Nazis eventually, whether in Star Trek or Doctor Who or....
There are also some indications that the author hasn't thought this whole changeling thing through: Miles and the others think that tying him up while he's unconscious (from being hit on the head -- would that really do anything to a changeling?) will keep him safe and secure. They learn otherwise, of course, but they should have already known that.
The end of the book may strike some more jaded and cynical readers as fanwanky, but I liked it. First, Miles arranges for Jake to meet someone Miles knows pretty well: Captain Jean-Luc Picard. Second, Picard reveals why he wanted to meet Jake: he has a letter one of his ancestors wrote after the war that the family has passed down until the right moment. Yep, that cute French girl was Picard's great-great-etc-grandmother, and she sends Jake a letter to tell him she survived. Once Picard enters the scene, the family connection's not hard to see coming, but it's a nice touch to end a series of DS9 books with an appearance from a guest star from the very first DS9 episode. And the letter, about surviving the war, strikes a chord for Jake, who's living through the Dominion War.
Overall, I liked it.