My favorite probability book is "Probability and Random Processes" by Grimmett and Stirzaker.
Some people consider this book to be "advanced" but I don't think it is that bad if you are comfortable with calculus, mathematical notation, etc. The exercises are non-trivial and they admit as much. I think you can see a pretty good chunk of it on Google Books if you want a preview.
If needed, use the Schaum's outline (Probability, Random Variables, and Random Processes) as a backup as it moves a little slower and works as a good guide to the first 4 chapters or so.