I should point out that it's not particularly long in text terms - "Bleak House" or "Middlemarch" are both much longer, and they are single books! The file is large only because of all the pictures.
I think I've pretty much mastered the technique of creating "omnibuses" from separate book sources now. I no longer use the "Join Books" tool - it has a habit of chopping books off in mid-text, and can't handle books with pictures where both call them "image1", "image2", etc. What I do is run two instances of BD - one for the omnibus, and one for the book to be added to it. I open the book I want to add in the separate instance of BD, then copy it and paste it into the other instance to form the omnibus. That way all the pictures come across properly even if the originals have clashing names for the image filenames.