Why buy otherwise free ebooks?
Recently many public domain eBooks have appeared as paid 'apps' in AppWorld.
Most, if not all, of these books are available as FREE ePub downloads from public sites - most linked through Internet Archive: ARCHIVE.ORG
AppWorld has several ePub readers which have a one time charge for about $2-5. Almost all are good enough to read these free books.
Are some purchasers of these paid apps just newbees who don't know how to get free stuff? .. Or, paid books simply do not have sales? .. Or, are some paid books in fact great? Yes, I can understand that a well illustrated book is worthwhile, but, not a text based book.
With these, the appWorld collection of 'apps' is building up fast. :)