I love
The Door Into Summer by Robert Heinlein. He's a classic American sci-fi author with a distinctive style - no idea if this is still in print, or if you'll be able to find it where you are. Sometimes I like him, sometimes I don't, but this book is him at his best. It's funny, there are paradoxes and revenge and a romance. It's all pretty light-hearted and a bit over the top, and probably you shouldn't squint too hard at the romance, but it's just so enjoyable an adventure and the twists are fun to figure out.
I look forward to other recommendations! I love time travel books too.
