The Jargon File (version 4.4.7, 29 Dec 2003):

daemon book
 n.

    The Design and Implementation of the 4.3BSD UNIX Operating System, by
    Samuel J. Leffler, Marshall Kirk McKusick, Michael J. Karels, and John S.
    Quarterman (Addison-Wesley Publishers, 1989, ISBN 0-201-06196-1); or The
    Design and Implementation of the 4.4 BSD Operating System by Marshall Kirk
    McKusick, Keith Bostic, Michael J. Karels and John S. Quarterman
    (Addison-Wesley Longman, 1996, ISBN 0-201-54979-4) Either of the standard
    reference books on the internals of BSD Unix. So called because the
    covers have a picture depicting a little demon (a visual play on daemon)
    in sneakers, holding a pitchfork (referring to one of the characteristic
    features of Unix, the fork(2) system call).