I am trying to complete my Ars Magica book collection and Realms of Power: The Divine is not available anywhere or has ominous 'out of print' messages. Am I doomed never to own this book? Not even Ebay has a copy
It was done as print-on-demand, with a source that's no longer providing that service. We expect to move it to Amazon print-on-demand, but that requires some format changes that aren't simple. So the answer is YES it will be available again in future, but I can't speculate on when.
Same thing here, except I'm also missing The Infernal.
Infernal is already up on Amazon as print-on-demand: