Server Fault (specifically) does not "do" learning material requests. We try to focus on questions that have a specific answer to them, and Where/How can I learn more about technology X? doesn't fit well. Unless "lists of resources" are well curated they turn to shit, and the value of the site is hurt by that.
A better approach would be to ask and answer your own "How do VPNs work?" question, in the style of our IPv4 subnetting or IPv6 subnetting questions. This makes Server Fault the focus for learning, rather than just an empty signpost saying "Go buy/read this material".
You would need to provide an answer of the same breadth and depth as the subnetting questions for this to be truly useful, which means you would have to do the legwork to find the resources and write up the summary (Chat can help within reason - we won't do all the research for you, but if you want to embark on writing this kind of in-depth answer and have questions I'm sure folks would be willing to help clarify any points you need help on).
Note that other Stack Exchange communities have different views on the subject.
Stack Overflow has allowed the C++ Book List to live, and Aviation is experimenting with the idea as well.
For Server Fault in particular we've found these types of questions tend to do poorly (because nobody wants to curate them), so we generally don't allow them.
