Hi,
@Emia,
I don’t know about written guidebooks, but I can tell you that the premier website for walking to Fátima has been de-activated. You can read about the reasons on their website.
Associação de Amigos dos Caminhos de Fátima. Divulgamos as peregrinações e promovemos os caminhos de Fátima e Santiago We promote pilgrimages and the ways of Fátima and Santiago.
The website does, however, still contain GPS tracks and a list of accommodations.
Following the coastal route “backwards” might be aa bit challenging. And I believe the Fátima arrows may also be somewhat sporadic. So I would definitely recommend using your phone to download tracks to follow when things get unclear.
Good luck with the plan! Bom caminho, Laurie