I'll answer my own question: I can reliably generate thumbnails in the Book menu if I use Calibre to generate .mobi files. Apparently, .prc files and .fb2 files are iffy and .epub and .pdf will not generate thumbnails at all. Is there any way to tweak the file manager of the PB360 to generate Thumbnails for all book formats? It's somewhat annoying to have run all of my ebooks, even the mobipocket format ones, through calibre in order to have a nice screen full of book covers. My old Sony and Cybook did this automatically without any jiggery pokery. The PB360, with its advanced firmware, should be able to handle this easily.