Though it's not by the same authors, isn't set in the same world, has no characters in common, and isn't even especially similar in tone, I can't help but think of Patricia Wrede and Caroline Stevermer's Sorcery and Cecelia as a companion to Brust and Bull's Freedom and Necessity.
The biggest reason for this is that they had the same writing method: The authors wrote in-character mail to each other (with no extra-curricular plot conferencing), and published the result as an epistolary novel. This leads, in both cases, to a slightly rambling plot with oddly disjoint pacing. The secondary reason for the similarity is that they're both set in 19th century England -- though the England of Wrede and Stevermer is one with Wizards' Colleges and working magic.
Still, they are very different books; the biggest difference is that Sorcery and Cecelia has younger protagonists, two teen girls who get mixed up in sorcerous affairs. This is, alas, the reason why S&C didn't work for me as well as F&N -- at various points in the book, the girls are stupid. They fail to put together obvious clues ("Oh, the woman who tried to kill you is named Miranda? Well, in completely unrelated news, a woman named Miranda came to town today."), and behave like the empty-headed ditzes they believe themselves not to be. I wanted to slap some sense into the silly little geese.
But when they're not being irritating, they've got interesting and distinctive narrative voices. Combine that with a setting I'm fond of (I almost always enjoy post-medieval fantasy), a pleasantly convoluted plot, and pacey writing; and it all makes for a very enjoyable read.