I read this without having read The Bear and the Nightingale. After only a few pages in, I knew I needed to read The Girl in the Tower NOW and made the split decision to see if it would work as a standalone novel, or if I could at least read them out of order(and necessity).
Vasya is a character that crawls right underneath your skin in all the right ways. She is fierce, independent, inquisitive, has some quirky magical awareness going on, and her stallion Solovey as a sidekick nearly did me in. I wanted nothing more than to jump on his back behind Vasya and run and run...except, that seat was already taken, first by Katya, followed briefly by Marya. I don't write spoilers, so you'll not get the explanation of the girls here...
The Girl in the Tower is much more than the second in a trilogy. Often, the second book does little more than bind the first to the third, but here instead I found the action to be intense, unrelenting, with little down time. And yes-I did purchase The Bear and the Nightingale almost immediately, and will gladly move it to the top of my TBR pile.