"The Tower of the Swallow" by Andrzej Sapkowski

The sixth book in the Geralt saga (or fourth, if you count only the novels), this book continues where "Baptism of Fire" left.  Some English editions have the plural "Swallows" in the title, but that's wrong, as the original Polish title is to be translated in singular in English.

Ciri is found by the hermit/professor/exile/philosopher Visygota and nursed back to health while she tells him her story of her band, the Rats, frolicking around doing banditry stuff, until getting murdered by the bounty hunter Leo Bonhart who was introduced by the end of the previous volume.  He is so meticulous that he saws off the heads of all the Rats after murdering them, slowly and neatly.  He captures Ciri and beats her viciously, leading her around on a rope around her neck and eventually forcing her to fight in an arena in Claremont for his and others amusement.

Geralt and his gang continues to search for Ciri.  They find Ciri's double Angulleme, who also joins them, and have a skirmish at some mines, where they barely escape with their lives, but continue to look for the Druids.  Geralt becomes much nicer to Cahir, who is scheduled to die in the next (last) book, together with most others, anyway.

Eventually they find the druids, but all is not peaches, as the druid(esse)s control walking, semi-sentient trees (Ents???) who capture all kinds of brigands but also Geralt and his gang, to be burned alive in a Wicker Man, a respected, age-old Druid tradition. Luckily Regis has some pull with the druidesses and persuades them to free their friends.  The gang then continues to Touissant, where the next book will take place.  Mostly. If you don't count a parallel universe where Elves exterminated humans.

Ciri gets better with Visygota's care and sets of on a quest to revenge herself and her friends, despite Visygota's strong protests. Eventually she finds Leo Bonehead, Stefan Skellen, Rience and the rest of the merry company who enjoy torturing people with white-hot iron and dismembering them while still alive.  Ciri leads the hunting posse on a chase towards a frozen lake, beyond which the Tower of the Swallow lies with its thousand portals to time and space inside.  On the lake Ciri kills much of the group, minus Bonhart and Skellen, including cutting off the fingers of Rience while he clings to the edge of the ice with her ice skates.  Then she disappears in the Tower and teleports to the world of Elves (not intentionally).