This is the current version of a short book about atoms that I am writing. The book begins with a discussion of automata models for infinite alphabets. Then, inspired by these automata, the book develops the theory of orbit-finite sets with atoms and computation on them.
At the current stage, I do not intend to add new material except for: fixing errors, an introduction, exercises and their solutions. If you find mistakes in the current version, please put them here.