I haven't posted in a while so I thought I'd chime in as well!
Opuscula Magica 1 and 2 are fantastic - and as 509 pointed out Number 1 is OOP but I believe you may still be able to get copies from Midian and sometimes eBay without too much gouging. Both of those give a good amount of insight into the praxis of the Sabbatic Craft. As it has been mentioned, M. Howards 'Children of Cain' is a good read and goes well with Sabbatic articles written and found in his magazine 'the Cauldron', in Starfire 1.4 and 2.4, Esoterra and Abraxas #1.
Psalter of Cain has beautiful content, imagery, and poetry and is printed using an old letter press style which in my opinion is amazing and is great to see a collaborative effort by the Cultus Sabbati being produced.
I actually received my copy of the Azoetia - sethos yesterday and am really excited to start digging in - but as it cost me a fortune to track down and purchase after many years I would recommend starting out with THP titles like Opusucla 1 and 2 (they're supposed to be published in a series of 5, so a lot more content will come out sooner or later), Iniatiation, Mysticism and Dream (OM#1).
I would also highly recommend the current Grant Magistar of the CS, D.A. Schulke's material - as it was said LVX HAERESIS is a great read, and ARS PHILTRON and VIRIDARIUM UMBRIS are in a league of their own.
I love Crowley, Grant and Spare and have become just as enthralled by Chumbley, Schulke, Fitzgerald & co. in recent years!
Looking forward to seeing more discussions on Lashtal!