Many people consider this to be the classic of the field:
Operating Systems: Design and Implementation, by Andrew S Tanenbaum. Prentice-Hall 1987, ISBN 0-13-637406-9
(Tanenbaum was also the guiding light behind Minix, which was sort of the Linux of the 1980s.)
A shorter book which also comes highly recommended (which I think I prefer) is:
The Design of the Unix Operating System, by Maurice J Bach. Prentice-Hall 1986, ISBN 0-13-201799-7 025
(This is another book of which I've wound up with two copies, which is some sort of testimonial.)
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