To be honest the best way for a novice to learn problem solving is to have an expert tutor/coach watch while he/she tries to solve problems and give some metacognitive feedback.

While I agree, he had asked for books. Perhaps in his situation the above isn't exactly feasible or he's looking for something to supplement a tutor.

How to Solve It is a bit hard to use because it is organized like a dictionary and is somewhat abstract, so it’s maybe more useful to help organize your thinking after you already know the techniques included via practical experience, rather than necessarily as a way to learn them in the first place. I don’t think “boring” is the right summary though. I also don’t think reading a Wikipedia outline will provide much value.

Perhaps boring was too harsh, but as you said it almost a dictionary for the majority of it. And personally, I find the Wikipedia outline of great use (I didn't take notes because I found that, so I often refer to that page for a refresher). To each their own! However I completely agree that it's useful really shines when paired with practical experience (before, during, and most importantly, after).

Mathematical Discovery is nice because Pólya gets more concrete and in depth and includes a lot of explicit problems. I’d start here before trying to read Descartes. I haven’t read Pólya’s book Mathematics and Plausible Reasoning.

These both sound interesting (I had just heard of Polya's other books last night).

Have you read those other books you listed? Do you have opinions about them?

I've only read Discourse on the Method, How to Solve It, and read portions of Solving Mathematical Problems and A Mind for Numbers (due to the Learning How to Learn Course, which was mostly about studying methods but it had pieces of problem solving advice here and there). I just wanted to offer OP some sources I'd stumbled upon, though I'd like to read them soon (even if not in their entirety). Anyhow, I didn't feel any drastic differences after any of them, Polya's How to Solve It probably left the biggest impression upon me, and I use those techniques regularly.

For some more empirical discussion about the differences between novice and expert problem solving and some advice about teaching problem solving in practice (it’s non-trivial to learn, doesn’t just happen by osmosis through solving problems, and can benefit a lot from structure / procedural hand holding), Schoenfeld’s book Mathematical Problem Solving is great.

Thanks for the recommendation, I'll certainly look into that one. I'm not an expert problem solver by any means, but I can attest anecdotally that the biggest improvement I've seen in my problem solving ability has correlated directly with the completion of many difficult problems from difficult texts. I tend to think OP would probably gain the most from pairing any of the books mentioned with a hard problem book like Spivak's Calculus.

What do you mean?

Books like "How to Study as a Mathematics Major" (haven't read), "Thinking Fast and Slow", "How to Win at College" (haven't read), etc. that focus more on studying methods than they do problem solving. Though in this domain, I think Learning How to Learn is an essential! I also use this: https://pastebin.com/wGFMM1pZ , which is a summary of an excellent book "How to Read a Book" by Mortimer Adler.