So yeah I think these are great ideas for Add a "blank book" to allow the user to track reading material not applicable to Natively but I think you’re also illustrating that this isn’t a super straightforward to add. There are quite a few things to consider. 
I do think that we should have a new content type for all these non-book formats like inserts or web comic. Anything that isn’t marked by chapters and pages should not be a book… should be something else. But I know that people do a lot of reading on these things so it’d be great to track. We just have to do it sensibly.
Doujinshi that are published in a more traditional book or things that do not have isbns but are a book could definitely be added as a ‘book’.
As a random aside… do we think a lot of users prefer character counts to pages? I wonder how useful it’d be to allow users to submit a character counts for books which could be used by for everyone. As long as they didn’t include a preface, they should be same across editions. Character count may become more of a thing as we add more item types, as it is cross format. Granted, I’m not in any hurry to add more item types… so hard to do well 
