Notebooks 8.2.3 (released March 26) addresses a couple of stability issues, especially when using Notebooks in iOS 9 split view. It also takes care of the glitch that the row of extra keyboard keys did not resize when the device is rotated while editing a document.
We finally found out what prevented Notebooks from successfully starting up on some iPads in portrait mode (it was due to the iOS accessibility option Speak Screen) and fixed that as well.
Notebooks 8.2.4 (released March 30) takes care of a glitch introduced in 8.2.3: documents which were imported during Dropbox sync were accidentally displayed as members of the current book, no matter where they actually resided. The document were stored in their correct location, so this really was just an interface hiccup.