The conversion of an epub to plaintext is quick and useful.
I do not see any use for the search if I cannot display/read the epub!
I am not aware of any Quicklook plugin that lets me read the epub. There are some plugins that let you see the cover and read metadata (a brief description).
(1) I index an existing books from iBooks. All I see in DTP is the book’s cryptic file name (see d.pr/i/TxO0+). It appears that I can, in fact, search into the book, but I cannot see any of the books’s contents.
(2) I installed Calibre as you suggested above. Whenever I try to load an iBooks-format epub book into it, it says it cannot understand the book’s format.
Am I missing something? It doesn’t seem as if the epub indexing feature is much of a feature.
I have used Calibre in the past and converted my ebooks to PDF so I am not tied into any specific ebook reader. Works fine for me with purchases from Kindle, iBooks and GoogleBooks. I then have the flexibility to import them into DTPO, read, highlight, etc to suit my requirements. One thing to note when doing this is the page size/layout needs to be set correctly to maintain similar formatting to the original device as shown here: