Coincidentally, I happen to be reading the String Theory trilogy right now. (For the first time. Just wanted to get it in before I read The Eternal Tide.)
I am finding the 3rd book a little harder to get through, as people have said here. I'm almost done and it's taken me about twice as long to read the third book compared to the first 2. My problem wasn't the science or technobable so much as the fact that it just seems more boring, with Voyager people not doing much and much of the "action" being directed by "higher-order" beings like the Nacene or Q (or q). Plus, Janeway is out of play most of the book.
The books take place between the 4th and 5th seasons of the show. Seven is still in here early stages of "becoming human", still learning to work with others after her recent decision in 4th season finale to back Janeway and stay on Voyager versus returning to the Borg. B'Elanna and Tom romance in early stages. B'Elanna still dealing with learning recently (late 4th season) that all the Maquis are dead. (It's a few episodes into the 5th season when Chakotay has to stop her from doing dangerous stuff on the holodeck, and the writers do a good job capturing her feelings in these books building towards that later episode.) Janeway getting reminded of the part she played in the whole Caretaker affair that trapped Voyager in the Delta Quadrant; this is a good novel lead-in to the first episode of season 5, in which she seems to be suddenly mopey about it for no reason.
1st book a good look at Torres and Seven.
2nd book (the one by Kirsten Beyer) has good stuff with Tuvok and Janeway in particular. Plus it has some good world building to set up situation for book 3.
3rd book has some interesting revelations about Ocampa, of all places. Plus, it has lots of Universe-altering stuff happening where we are minor, but altogether important, players, surrounded by much more powerful and ancient beings.