Metal is fairly specialized knowledge partially due to it being a proprietary API only available on Apple platforms. Furthermore, most Apple developers use higher level frameworks for drawing to the screen, so there is not a huge demand for tutorials or books about Metal.
Still, there are a few resources which are available:
There are also samples and other notes in Apple’s archived documentation. Metal usually has at least 3 WWDC sessions every year so it is still an evolving technology that Apple is investing in.
Also, I’ve found 3 books related to Metal:
If I were to choose one, I would pick Metal by Tutorials. It gives the basic information to start building many game rendering engine features. The book can be a bit overwhelming because it usually gives a brief introduction to a rendering engine concept and then shows a code example through a mini-project. However, if you do not understand the concept, it can be hard to follow the code, so you may need to use other resources. The book does provide other references for further reading. The book has been recently updated for Metal advancements up to iOS 13 in 2019.
I hope to have a more robust list of math and general graphics programming books in a future blog post.