@mwl @drrjv
Not a book exactly, but my decades of notes on securing servers and setting up postfix may as well be!
These days, given an empty debian vps, rDNS and less than two hours, you can have an instance of any/all of several fedi things running with a further few mouse clicks using YUNOhost. During the initial setup, you end up with postfix, clamav, dovecot and roundcube webmail by default.
I'm still giddy with the simplicity every time I set one up.
Edit: fine for a hobbyist or very SMB, not a big org or important production.