This textbook is a bit heavy but worth it for the amount of content inside.
Signals and Systems by Barry Van Veen and Simon Haykin 2002 Hardcover Book
This textbook is well organized, with each chapter building on the last one.
Signals and Systems (2nd Ed) by Alan Willsky, S. Nawab and Alan Oppenheim
A nice touch in this textbook is the self-assessment quizzes after each lesson.
