You can get $5 O'Reilly EPUBs from the App Store everyday.
Extracting an epub from an iPhone app is easy. First you unzip the .ipa file. Then you cd to Payload/*/book. Finally, you zip up the files to an .epub file: zip -X ~/book.epub mimetype && zip -r9DX ~/book.epub META-INF OEBPS.
mimetype must be first in the archive for validity, which you can test with EpubCheck.