This book explores general signal and system concepts and characteristics for both continuous-time and discrete-time signals and systems. It progresses from signal representation and characteristics to the analysis of the effect of systems on signals.
Please, don't ever spend money to buy this book if you are trying to learn about signals and systems. This book is full of mistakes that are hard to catch if you don't have a professor telling you everything that was incorrect in the book. It is also extremely poorly written, so it is hard to encourage yourself to sit down, read and enjoy it. I'm not sure what other books to recommend you to so I apologize for that. If I ever find out, I'll try to remember to edit this comment with my other recommendation.