I've said that CoolReader stores epubs in the cache since for the user it looks so and nothing changed in this regard in the 1.7 version except for location of the cache directory. But actually it doesn't store the epubs (it wouldn't make much sense). CoolReader stores its representation of the epubs so that it doesn't need to reparse the file, split it to the pages next time you open it. That makes opening the file for the second time much faster than the first one.
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