Schott Neue Jazz-Harmonielehre

Because this book is incredibly densely written, parts can be rather difficult to get through as a hobby musician. Definitely recommend having a piano (or your instrument of choice, but it seems a lot easier with a piano) close by to get your ears to understand what Sikora is saying with the practical examples provided. Sometimes it is a little bit tough to get through, and I've only just covered the first 100 pages despite owning the book for a few years now - but I am 100% happy with my purchase. If I am ever stuck with a piece or want to expand my knowledge, this book is usually my go-to way to improve. Highly recommend!

TL;DR very densely written, often highly theoretical, but has everything you will ever need for applying music theory with examples to play yourself. If you need a more practical approach with theory phrased in more digestible bits, this may not be the book for you.