@wildstars Thanks for doing the comparison. As for what you said about deflate vs. store (no compression):
We don't compress images in our EPUBs now because it provides a performance improvement when loading the EPUB in e-readers. JPEG is already optimally compressed, and adding deflate compression on top of it serves no good purpose. If you store an image without compressing it, the e-reader has to spend less work and memory loading the resource.
Regarding the EPUB spec, first of all, the EPUBs you download are 100% compliant with EPUBCheck, the EPUB compliance checker, which I have double-checked for Mapping V3. You are correct that if a resource in an EPUB is encrypted, you need to specify whether or not it was compressed in a metadata file in the EPUB, but our EPUBs are DRM-free, and as such the compression flag is natively available in the ZIP format for any e-reader. If you check section 4.2 of the EPUB3 container format spec you'll see OCF ZIP Containers MUST include only stored (uncompressed) and Deflate-compressed ZIP entries within the ZIP archive.. Therefore, storing without compression is perfectly valid, and I am inclined to say the ball is in FBReader's court, assuming there is nothing else wrong with the EPUB.