I actually just tested this out with a snippet from Small Gods. It still sounds very robotic and, in my opinion, is nowhere near good enough for these books.
The timing between sentences and paragraphs is inconsistent, the intonation for sentences with ellipses is completely wrong, hyphenated sentences are treated as continuous strings, and it can't get names of characters or places remotely close to their intended pronunciation.
It might be fine for reading a news article, but it is nowhere near ready to perform an audiobook.
Gmail labels are great but they're not universal, and are easy to strip out.
A lot of sites:
Don't allow +'s in email addresses
May let your register but then not login
Are aware of labels and simply strip them out
I have an email address I have only ever used with labels but still get spam to the non-labeled address. Spammers and email harvesters are very much aware of this trick, so it only works on legitimate sites.
"this hill was so easy to walk down, why is it so hard to walk up it?"