As university is a bit slow, we organize a group activity of doing math exercises (after learning the basics of the subject separately). Thus I am looking for a book with many exercises that are exciting and challenging enough to do in a group.
As an example, last year we did a section on topology from "Real mathematical analysis" by Charles C. Pugh. It had 145 problems ranging from basic to ones he can't solve himself.
The level is mostly undergraduate. It would be best if the books didn't assume much prior knowledge and started from the basics as we have people of very different backgrounds. The books are expected to reach something interesting though.
We are not picky about the subject. Anything could work, if the right book is found. We are slightly biased towards complex calculus, although I fear most of the exercises in that are mechanical.
In short, share books with fun exercises.