Operating Systems Three Easy Pieces 110 Remzi H Arpacidusseau by Remzi H. Arpaci-dusseau, Andrea C. Arpaci-dusseau instant download after payment.

This is the "Downloadable PDF" version of the book from Gumroad, version 1.10. A quote from the author about this version - "this is a nice convenience and adds things like a hyperlinked table of contents, index of terms, lists of hints, tips, systems advice, and a few other things not seen in the free version, all in one massive DRM-free PDF." The few versions I've seen already uploaded aren't the actual e-book PDFs to my knowledge. They're duplicated and/or contain articles that are site-only, leading me to believe that they were converted from the site articles.
The book does not contain the hardcover image (the comet/asteroid cover). Instead, it just lists the title and author of the book in text.
A description of the book by Lulu, a seller of the hardcover.
"A book covering the fundamentals of operating systems, including virtualization of the CPU and memory, threads and concurrency, and file and storage systems. Written by professors active in the field for 25 years, this text has been developed in the classrooms of the University of Wisconsin-Madison, and has been used in the instruction of thousands of students."