"Operating Systems: Design and Implementation" is a classic book that was walks through Minix. It was a commonly used as a follow up course with projects to mod

"Operating Systems: Design and Implementation" is a classic book that was walks through Minix. It was a commonly used as a follow up course with projects to modify its code. Due to the author wanting to keep it dedicated towards education rather than real systems, it did not become a professional project but did inspire Linux' creation. There are similar books for other OSes, such as BSD and Solaris. A broader but less detailed approach for casual readers are the books "Beautiful Code" and "The Architecture of Open Source Applications".