http://pastie.org/private/q3pmenzgtwytcqb8nshptg

Here's the long overdue honto tool I promised. First of all, many thanks to cbquaden who helped me with the decryption.

You need to know your hardware ID to use it. I didn't figure out how it's generated because there's an easier way of obtaining it so I didn't bother. Download Fiddler, run it, go to Tools -> Fiddler Options -> HTTPS, check everything, launch Honto app and log in. Choose the /download/myListInfoDownload request, click Text View in the upper right window and copy the deviceId value, should be all uppercase. You only need to do this once.

Python 3 and pycrypto are also required, go to Python folder, Scripts, run pip install pycrypto.

Spoiler
There are some limitations - as honto delivers books in many different formats, this script currently only decrypts epub ebooks which seems to be the most popular one. If the book folder has a _DIVF4 suffix (_DIVF2 might work too, but I haven't encountered any books like that) you can use it. Other formats I've seen are:

_BOOK - proprietary T-Time format. The DRM is different than epub books, currently taking a look at it. Even when decrypted it's completely useless since no converter to any sane format exists, unless you want to download T-Time reader and take screenshots.
_XMDF - looks like another proprietary format, probably the same decryption process as _BOOK.
_EPUBX  - proprietary McBoox format, surprisingly the files are not encrypted (at least what I've seen) - you can open them like a normal zip/epub and see what's inside, still it doesn't look like anything supports those.

If anyone could point me to free books on honto in other formats so I can take a look at them, I'd be grateful.