This frustrates me too. You can get, say, books 80-100 (or whatever we're up to) in the Deathlands series in eBook format, but not books 1-79. They're not all out of print though - or, at least, Amazon still sells PB runs of them, for what that's worth - so it seems like renegotiating would be possible.
I've terribly concerned about abandoned works.
Here's another thing I've noticed that is a real kick in the teeth: regional restrictions. You can get a lot of Margaret Atwood books in eBook format, but not - apparently - "Alias Grace"... unless you're in the UK. And I know that a lot of non-US MR users suffer from regional restrictions as well. It's all very frustrating and seems unnecessary.