Monstrous Regiment by Terry Pratchett

This book starts with a war in Borogravia--not that that's anything new--there's almost always a war on in Borogravia. But this time, Polly Perks is in on the action as she has joined the army to bring her brother back. Polly, now known as Ozzer, slowly realizes that she's not in an entirely normal regiment when someone hands her an extra pair of socks in the latrine. And when the women finally start getting honest about going to war, Borogravia finally has a chance of a better life for all of its citizens.