Back when I was working on my toy OS I also bought the book describing the implementation of an OS called MMURTL, apparently the book and the code are now available for free online: http://www.ipdatacorp.com/mmurtl/mmurtlv1.pdf http://www.ipdatacorp.com/mmurtl/ I liked it and found it quite useful but I remember the folks at alt.os.development being quite negative about it because of the design.
Also, Podanoffsky's 'Dissecting DOS' is a good book though obviously focused on, eh DOS.
https://pdos.csail.mit.edu/6.828/2019/xv6/book-riscv-rev0.pd...
CompTIA Linux+LPIC-1 Training and Exam Preparation Guide (Exam Codes LX0-103101-400 and LX0-104102-400); Asghar Ghori