For advanced C, should I read books or follow project tutorials?

Use both, but let the book or documentation explain the ideas and use projects to make them stick. A tutorial can show a working result without explaining why a particular pointer, memory layout, or build choice makes sense.

Read a small section, then write something of your own that uses it. K&R is a common starting point, though it is not a complete guide to modern systems programming.

Projects are where the gaps show up. Just do not copy along line by line and count that as learning. After a guided project, close the video and rebuild the main idea from scratch, or change one requirement so you have to make your own decisions.

Compile with warnings enabled and use tools such as a debugger and sanitizers. They can expose mistakes that a tutorial may gloss over.

First pin down what you mean by low-level. Embedded C, operating-system work, and security programming overlap in places, but each brings its own concepts beyond the C language. Pick a small project in the area that interests you, then read the relevant documentation when you hit a question.

A book gives you room to understand the language; a project gives you a reason to learn the surrounding tools and systems. Neither has to be the one true route.