Ebooks are tiny until the library manager converts them
Kindle and Calibre both keep local copies and generated formats. Where each stores them and what is worth keeping.
A text ebook is a megabyte or two, so a library of a thousand books is a couple of gigabytes at most. What changes that is conversion and comics: a library manager keeping several formats per book, and any illustrated title, which is images rather than text.
Where each app keeps its library
du -sh ~/Calibre\ Library ~/Documents/Calibre\ Library \
~/Library/Application\ Support/Kindle \
~/Library/Containers/com.amazon.Kindle 2>/dev/null | sort -hConverted formats are the multiplier
A library manager that converts on import ends up with the original plus each format you have generated, per book. Three formats for a thousand books is three libraries. The manager's own interface lists formats per book and can remove all but one in bulk, which is the tidy fix.
Comics and illustrated books
A comic or a photography book is a folder of images in a container, and a single volume can exceed a hundred megabytes. On a library where those are a minority of the titles they are usually the majority of the size, which is worth checking before doing anything else:
find ~/Calibre\ Library -type f -size +50M -exec ls -lh {} + 2>/dev/null \
| sort -k5 -h | tail -15Downloaded copies in reading apps
Reading apps download books to the device and keep them. Removing a download in the app keeps the book in your account, which is the same distinction as Remove Download in Books and the other Apple media apps, and it is the reversible option.
Backing up rather than deleting
A library of purchased or converted ebooks is small enough to fit on any external drive and is genuinely hard to reassemble, particularly if it includes files with your own annotations. Moving it rather than pruning it is usually the better answer.
Common questions
Why is my Calibre library so large?
Usually converted formats, one per book per format, and any comics or illustrated titles, which are images rather than text. The library interface can remove all but one format in bulk.
Where does the Kindle app store books on a Mac?
In its container or application support folder under ~/Library, depending on the version. Removing a download in the app keeps the book in your account and frees the local copy.
How much space do ebooks use?
A text book is one or two megabytes, so even a thousand of them is modest. Illustrated books and comics are the exception, at a hundred megabytes or more each.
Should I delete or move an ebook library?
Move it. A library with your own annotations and converted formats is hard to reassemble, and it is small enough to fit comfortably on any external drive.