I wish to study stochastic processes in my free time. Since this is just as a hobby, I wanted to go with a cheap textbook. In my endeavor I have found four candidates within my budget of 40$.
I have the basics of statistics and probability but a much stronger calculus background to go with it (Tensor calculus, etc.). My interests are more theoretical than practical and I won't shy away from mathematical formalism, rather that's where my curiosity lies. I would like a "medium" level of mathematical rigor, providing satisfying justifications but not getting bogged down too hard into the details.
I would like to know which of these might be recommended and why?
Introduction to the Theory of Random Processes by Gikhman & Skorokhod
Stochastic Processes and Filtering Theory by Jazwinski
An Introduction to Probability and Stochastic Processes by Melsa & Sage
Stochastic Processes by Parzen