Edits are generally used for fixing typos, adding in info from a link, fixing formatting etc. New information is sometimes added but it is generally not too much new information, maybe an extra quote or something to back up the answer. Adding a source for the OPs claims is also fine. Anything more than that and the editor has a few options.
Edit it in anyway - after all it's SEs goal to build knowledge and adding info is always better than not (if it is relevant).
Comment on the answer - let the answerer decide if they want the detail in their answer or not.
Post a new answer - whether community wiki or not (you decide, depends on how much work you put into it etc etc) adding the information somewhere in the relevant place should always be encouraged rather than not adding it.
Leave the information out - I'd always encourage to add information if it is relevant but maybe it's tangential or only confirms the OPs answer in another source so isn't entirely needed. I'd still suggest to leave it somewhere on the post but it may not be necessary.
Story Id Case
Story Id's are a bit different to the general case in that a lot of the hard work is actually done in getting the right answer. Here I'd be more liberal with adding in information yourself.
If you have access to the source material edit in relevant quotes that match the OPs points.
Link/title only - add a blurb or summary from somewhere.
You feel the answer ignore some of the OPs points but can see how they match the work, add in the detail.
OP describes a cover but there isn't one in the answer and you know of a matching one, edit one in.
Here I reworked the sentence to read as it was meant to (though looks like I forgot to fix it completely i.e. "one the the" should have been "one of the" but I must have missed it). I then edited in a link to the series as it provides more information for the OP to see if this is the correct story: covers, reviews, brief description etc.
Lastly, I edited in the summary from the link, it isn't the best summary in the world but it does give us some useful information not already included in the answer.
It's part of a series.
The place the world is set.
The story is about three characters.
The over arching theme between the books i.e. love.
It doesn't give any specific details but a general overview along with the place name might be enough to jog the OP's memory.
I edited in some information from the link, the direct quote, in case it goes down. This appeared to be what the OP was basing their description of the character off of at the time and it's always better to have that information in the answer itself.
The OP disagreed with my edit and rolled it back, that's fine it is their answer to do with as they please. However, they then added in a quote from the book with a similar description of the character so my edit must have been along the correct lines in the first place.
Lastly, to provide a bit of context to why these posts were edited in the first place. I am currently going through old posts and fixing tags where they should be fixed. At the same time as doing that I am trying to fix everything I see that could be done on the whole post, to avoid the post being bumped a few times down the line.
To put people at ease I am trying to do around only 5 or 6 posts a day, depending on traffic levels on the homepage, and spread them around the day. Some days I'll do more, some less. Don't worry though I won't be destroying the homepage with old posts.
If anyone does have a query about a specific edit I would gladly discuss it with them. I should always be pingable in TREU and commenting on the answer is another way you can reach me.