[davidfor]

Originally Posted by varaonaid

Sorry for the sort of generic title but I couldn't figure out how to distill my question to be short enough to work for the post title.

So, here's my situation. I have ebooks from different stores, Project Gutenberg, even a few really old MS Reader books, etc and therefore they are in all different formats. Some books are already in multiple formats per title but many are not. I don't want to overwrite any files which already exist in the correct format. So, is there a way to be able to get Calibre (either directly or via plugin) to be able to find and convert every mobi/azw3 book without, for example, an epub version already? And vice versa for epub missing mobi? Even if I could just search for these books first, then convert, rather than all in one step? I just mentioned these two formats to start with as they are the most prevalent and I know PDFs can be much more tricky to convert (and potentially dependent on what device you want to read it on for the most readable outcome). Plus, PDFs are somewhat more ecosystem agnostic than the epub/mobi (meaning that, for example, iOS cannot read mobi directly but can read PDF, and the same for other ebook apps or devices that can read primarily epub or mobi).

This is really: How can I find all books in my library that don't have a specific format?

In the tag browser, find the "Formats" entry. Expand this and find the format you are interested in. Click on it, the icon will change to a plus and calibre will show all the books with that format. Click on it again, the icon changes to a minus and calibre will show all the books that do not have that format.

You can also use ctrl-click to select more than one the format. If you look in the search bar as you do this, you can see the search that is created for you. There is an option near the bottom of the tag browser to change how multiple selections are handled - using using an "or" or an "and" to join the parts of the searches.

Once you have the list of books without the desired format, you can select them and do a conversion.

I'm trying not to litter up the forums with multiple questions at the same time by asking these here. But if this is too long to practically get an answer, please let me know and I can split it up.

I think you would have been better to split them. The questions are not really related. Asking them separately will lead to more focussed discussion on each of them. And it is likely that someone will read the first part and decide they can't help so won't read the rest. And I can't help with them, though I'm not sure I understand the second question.