Perhaps I'm misunderstanding your question based on the always helpful other users who have responded, but just in case I'm right.....
Assuming you are essentially "starting over" on your latest Calibre...
You've asked how to import all the books in a single folder even if it has subfolders.
I do this all the time.
Simply:
- open Calibre,
- hit "F" (same as Add Books, 3rd option down)
- then navigate to the highest level folder (in your case My ebooks),
- Double click your highest level folder (My ebooks).
- Click Select Folder at bottom.
All formats that you have Calibre set up to read will import. I'm not sure if other will import even if Calibre isn't there default for them. I'll assume since you used them in Calibre version before this isn't an issue.
When you double-click your highest level folder, it may look like you need to do more. What you see is any subfolders; you do not see files. That is ok. Select Folder means you are selecting whatever you've double-clicked.
There are THREE CAVEATS to this....
- Metadata: If you did not save any customized metadata in individual files, that will be gone. If a file is in a single folder with an .opf file, you will likely have your metadata.
- Custom Column Metadata: If you had any custom columns when managing the books, you will need to recreate those custom columns exactly according to the lookup name BEFORE import. Otherwise, that info will not import.
- Be sure to check your Adding Books Preferences before doing this.
"Thousands of books" can be a few to tens of thousands. I
highly recommend so as to avoid lock-ups that you break up your My ebooks folder into at least a few primary subfolders, then do this process to each of those. It will import faster in my experience as that is a lot of files that have to be written by Calibre. It will also let you import a few hundred at once after checking the caveats above and making sure it is doing what you want without waiting for hours (assuming not lockups) for everything to import at once.
I hope this helps.
P.S.: As someone who used to manage my own files and folders prior to finding Caliber and then thought Caliber would just help me fix metadata and not manage them, let me also highly suggest your use Caliber to the fullest potential of management. I discovered it's power for a LARGE library and only regret not using it a year or two prior when I first saw the name. Then, you can always recover your library easily. Also, should you want to "grab some files" with individual names and formats (Author-Series 01-Title.epub or .mobi), you can always use Save to Disk. I do this all the time to send to a custom folder used for my ebook reader.