Greece on Foot – Mountain Treks and island trails by Marc S. Dubin is of course the basis for most of the stuff here as it is sadly out of print but full of useful information.
His second effort combined efforts with Michael Cullen in the venerable “Trekking in Greece” guide which went even further but is also rather hard to find anymore.
People just don’t read books anymore and the internet or mobile devices are now standard equipment even with backpackers. But somewhere on your travels you will want something more to read and here you will find ideas relating to Greece.