@rdmx-
You see!
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When you add a book to your PW, it will index all of the words so that if you do a search in the future it can take you to it. Sometimes when you add a book to your device it can be corrupted and then your PW will get stuck (choke) while trying to index it. This is why it is recommended to not add a lot of books at one time because it can be a pain to figure out which is the problem book file. To check, do a search from the home screen of your PW on a nonsense word like "pplkj". If you see that it is still 'indexing' a book and you haven't added one recently, then you need to delete that book from your device and do a re-conversion of the book in Calibre before re-adding it to your device.