The biggest problem I've found with PG texts is that some of them have lost information contained in the original source book, e.g.,
Italics--many PG texts represent italics using UPPER CASE, which confounds real upper case with italics. For example, the following source will all be represented the same in the PG text (as HELLO WORLD):
No "smart quotes". Opening and closing quotes are represented by the same character, sometimes a double quote, and sometimes a single quote. When single quotes are used, this is confused with apostrophes, which are usually represented by single quotes.
Confusion between hyphens and em-dashes. Hyphens should be represented by "-", and em-dashes by "--" in the PG text, which is easy to convert, but this is not always the case.
In my own private utilities I fix the above: convert to using opening/closing double quotes, correct apostrophe characters, correct use of italics etc. I do this automatically for many cases, and for the more difficult ones, I prompt the user for their resolution (what the correct character should be).
I wonder what your thoughts are on this and how it applies to Gutebook?