Encryption is a waste of complexity and battery life here.

Firefox encrypts your history and open tabs because those are sensitive.

A page number in a book is not.

But they don't only sync page numbers, but also written annotations (and bookmarks and highlights).

And regarding battery life, even a CPU much weaker than the one used by the original Kindle can encrypt at 1MB/s, so encrypting a couple of KB is absolutely negligible.

A detailed history of what pages of what books I'm reading and when is certainly as sensitive as browser history, if not moreso.