[zozbot234]

Legally speaking, yes they would definitely be. Many old books and periodicals about retrocomputing can now be readily accessed via the Internet Archive and other efforts such as Bitsavers because no one really cares about enforcing these copyrights (and even finding the current right-owners might be infeasible), but selling newly-made hardcopies commercially is way more of a grey area, AIUI.

(There might be some ground for challenging the copyright for works that were first published before 1989 without a copyright notice. But I'm not sure that would apply to these guides.)