I’ve done some more research and I think I found it. It’s already out of Print and I think never that big to begin with, but still I present:

On Her Majesty’s Arcane Service

Depends ok the DRM technology. The only PDF DRM I came across so far was using the horrible feature by adobe digital editions.

It can be removed by using calibre and an unofficial plugin called noDRM, which should work on Mac as well (didnt test it tho, I dont own a Mac. Worked fine on windows tho)

Dream Theater and Classical music have entered the chat

Was actually confronted with the same problem - thank you for the solution!

Aegis authenticator. Beats all proprietary apps I’ve tried so far