I had the joy of discovering AMS'Student Mathematical Library book series today, and I have been pleasantly surprised by how enticing some of the titles seem: exciting and expositionary, a perfect stepping stone for learning!
I am familar with some Springer book series (Undergraduate/Graduate Texts in Mathematics), but I think those have a much more of a textbook nature in general.
What are some great book series that fit the style of Student Mathematical Library?
See this question for inspiration as to what the answers should look like.