Eli Bendersky's website This book began as MIT course notes when the author was a TA in the course. In it, Mr. Karu claims to explore the topic of Signals and Systems in a very clear manner.
The scope of this small book (120 pages - compare it to the textbook monsters !) is quite big - a bit larger than a standard Linear Systems course as I know it. Discrete/continuous systems, laplace analysis, phase coordinates, fourier series/transforms, (de)convolution, filters, sampling, simple feedback, etc.
I have mixed feelings about this book. On one hand, it doesn't fulfill it's promise, IMHO. The explanations are far from "ridiculously simple" - hardly any motivation is given and the examples are very sparse. Material is pumped into the student in a high pace - just like a Uni course. So, I guess that the secondary title should've been just "Course notes".
On the other hand, for me this book served a perfect purpose. Having studied this stuff in Uni, I don't need "ridiculously simple" explanation, but a packed refresher that covers a lot of material in a short time. For this - the book is perfect. What's even better is that most of the examples are from electric circuits - just what I needed !
So, get this book if you need a tight refresher/reference on the topic. DON'T hope to study Signals and Systems from it for the first time.