Hi David, thanks for your feedback!
I checked your feed and could see what you mean. You are right, in the case of a mixed feed (catalogs and books), both are handled as books. I checked other pages of your feed and saw, that in the case of authors you don't send a mixed feed. In this case you send first a catalog with all letters, including those with just one author and in the next step the single authors are listed.
I checked also about 10 other OPDS feeds and couldn't find another one sending a mixed feed. Or did I miss one?
So I wouldn't say, that this is a bug, EPUBReader just doesn't support this ;-). If the situation changes and there are other OPDS feeds sending mixed feeds, I'll think about it.