Perfect Update!

I love the new sync feature (v3.9)! I have two actively used iPads shared among my family members each with a different Apple account id and his or her own iPhone or iPod. I prefer its sync model over those of iBooks and Kindle for exactly those reasons highlighted in its Whats New section.

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Aug 22, 2013:
This update (v3.4.2) is very very stable. Ive updated the app on all my new and old iOS devices ranging from those running iOS 4.3 to iOS 6.1.3. In fact, I have downloaded every single update since v3.1. Not a single book has ever been lost as a consequence of ShuBook update. I love it! This is much much better than some other similar apps track record.

Moreover, I have some general advice to other users. The apps description has indeed warned that there is no guarantee that the books can be restored after an iOS system upgrade. (Some other apps that also let you download a huge number of books and files from third-party sources also have similar warnings.) However, when I do in-device upgrade rather than upgrade via iTunes, all my app updates are flawless and faster. Avoid doing your system update via iTunes!

Huang Shan about ShuBook 1F