[1] https://news.ycombinator.com/item?id=14161876
How to solve it by Polya - an awesome book. Read these to learn not just mathematical problem solving but in general. I don't think I've read a book quite like this before. The idea of a process and tools for problem solving is awesome.
How to prove it - a great book. I felt after reading through this I had a much better understanding of how proof works. I also had a better understanding of how mathematicians go from scratch work and proof to the proofs they finally present. There's a lot more that goes behind them and will help you understand those proofs by giving you the tools to do the scratch work behind them yourself. I had tried to learn proof techniques from a few other sources, including Jeremy Kun's awesome primers.
"Conceptual mathematics" - did a very good job in helping me make more progress with category theory. I tried a few different books and an online series. Flicking between those and this book, I started to understand much better.
The Little Schemer - the one that finally opened the key to recursion, closures and functional programming. I had tried attacking this using a few other books, including SICP but TLS helped a tonne in the final clicking.
Thinking in Type - great book that taught me quite a bit about typography. It showed me how important typography is to design.
You can draw in 30 days - sounds very SAMS but actually the progress I made in 1 day is testimony to the book's bold claim. You won't be Da Vinci but you will have functional drawing and sketching skills if you go through this.
It's important to note my process in finding these books. I think much of the groundwork and mental priming before working through most of the above helped a lot.
A tend to get interested in a topic and then seek out good books in that field. If the book doesn't make much sense or is very dense, I keep at it for a day or so. I then seek out similar books and repeat the process until a little jump forward.
Unfortunately in around 1 to 2 weeks, I've lost interest and I repeat the same thing on a different topic.
I probably have a tonne more but they're spread across many subjects so it's difficult to consolidate. Feel free to email me if you want recommendations for a particular subject.