crdf,
The question you asked cannot be answered directly because it depends on the books that you want to add. There are three ways to do this:
1)With some books you will get valid metadata from inside the ebook file. For other books, the metadata inside is useless. There is no way to know for sure without looking inside the book (which might be too advanced for you just yet) or simply adding the book and seeing what you get.
2) Other books have the metadata incorporated within the ebook's filename. There are several different naming schemes that are in use, so adding these books requires an appropriated regex for each naming scheme. If the regex doesn't sufficiently match the scheme, the metadata that is imported will be partially or totally wrong. Regardless, any variation(s) in data will result in non-conforming records (like the author issue you've already seen).
You could take a look at this older topic,
Tyrannosaurus Regex for a regex posted by me (but created by multiple people) that is pretty good at handling an
Author - Series Series_Index - Title filename scheme. That is a very complex regex. There are similar regexes for other schemes posted in here somewhere.
3) Finally, it is possible to obtain metadata from the internet from a variety of websites. This requires getting the Author, Title and (usually) the ISBN metadata correct before you can make use of this feature. Generally this should be done slowly and carefully until you are more familiar with the feature as it overwrites metadata.
None of these methods are perfect. You will want to manually edit your book records to arrange things to best suit your needs.