It's happened to me twice. Once in a periodical and once in a large book (1700+ pages). I was using the device daily for a week and read one book (purchased with DRM) and one short story (borrowed from library) and have been reading another short story (public domain - no DRM) without any issues until it froze yesterday. Then today I had a weird issue where the text was all jumbled up. It was turning pages normally but text from the previous chapter was inserted in the middle of pages in the new chapter. I had to exit the book and then reload the current chapter.
I have no SD card and about 250 books loaded now. The one thing that did change was that until this weekend I only had about a dozen books loaded. The freezing happened after I loaded more books. Could be related or could just be coincidence...