"The best introductory text we have seen." � Cosmos. Lucidly and gradually explains sets and relations, the natural number sequence and its generalization, extension of natural numbers to real numbers, logic, informal axiomatic mathematics, Boolean algebras, informal axiomatic set theory, several algebraic theories, and 1st-order theories. Its clarity makes this book excellent for self-study.
This is the most didactic introduction to Set Theory I have found in print. All definitions have attached non-trivial and interesting examples. The exercises are well-organized and chosen with care. It often uses terminology that is not commonly used in the 2020s as it was in the 1970s (the book's latest edition is from 1979), but these issues are straightforward to navigate.