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Some titles from Overdrive are only distributed in Overdrive Read or Kindle format. The title must be in EPUB format to work in SimplyE. There are a very low number of titles that aren't available in EPUB. The titles that aren't in EPUB format still come through Overdrive's API and are delivered to SimplyE. To clear these out there is a scheduled task that runs to make sure that the collection that you see in SimplyE only includes formats that are supported by the app.
What do the "Main Collection," "Popular Books," and "Everything" facets mean?
The "Main Collection" facet was created for a specific purpose, e.g., to filter out thousands of non-classic works imported through Project Gutenberg. These works were swamping search results with books that held little interest for contemporary readers, so NYPL created a facet to exclude them. As far as we know, NYPL is the only library that imported these works into its collection before the project replaced it with Project Gutenberg's public domain ebooks with CC-licensed editions from Feedbooks. Most of these titles were removed from NYPL's collection, therefore the need for "Main Collection" no longer exists. This face should be removed with JIRA ticket SIMPLY-2286.
The "Popular Books" facet includes titles with a high quality score, which combines the common-sense notions of "literary quality" and "popularity" into a single number. By default this number is .65, but it can be modified by the library in the Circulation Manager.
"Everything" includes all titles within the collection.