[WhiteSage]

For complex analysis the book by Churchill -written for engineers- is a very down to earth introduction with lots of examples and applications to resolution of integrals and PDEs.

In the case of set theory I cannot recommend enough Halmos' "Naive set theory". It's completely rigorous yet easy to read, and lays out all the basics of ZF, constructing both ordinals and cardinals.

For probability Khinchin has some very thin fun books, written for some classes he gave in a military school. The books do not get into deep topics but are a good introduction.

For group theory I would try Isaacs' Algebra book. Although the book is one of those books which contain too many topics, he was really enthusiastic of group theory and I have been told the corresponding chapters are very good.