Audiobooks are the largest thing the Books app ever downloads
An ebook is a megabyte and an audiobook is a gigabyte. Where downloads are kept, and how to remove one without losing the purchase.
Books is easy to overlook because the app is small and most of what people read is small. Audiobooks are the exception, at roughly a gigabyte for a long one, and they download to the Mac by default if you have ever opened one there.
Where they are
du -sh ~/Library/Containers/com.apple.BKAgentService \
~/Library/Containers/com.apple.iBooksX* \
~/Library/Group\ Containers/*bookassetd* 2>/dev/null | sort -hLike the other Apple media apps, everything is inside containers rather than in a folder you would find in Finder, which is why it lands in System Data on the storage screen with no explanation.
Removing a download without losing the book
In the Books app, right click an item and choose Remove Download. The book stays in your library and downloads again when you open it, exactly like the equivalent option in Music and TV. Choosing Delete instead removes it from the library across your devices, which is a different and much larger action.
That distinction is the entire risk here. The two options sit next to each other in the same menu, and one of them is reversible while the other is not.
PDFs and imported files
Books also holds anything you dragged into it, including PDFs, and those are not redownloadable because they were never purchased. If your library contains imported files, export or copy them out before removing anything in bulk.
find ~/Library/Containers/com.apple.BKAgentService -type f -size +200M \
-exec ls -lh {} + 2>/dev/null | sort -k5 -h | tail -10Whether it is worth doing
For most Macs, no: a handful of ebooks is a rounding error. For anyone with a habit of audiobooks it is a few gigabytes that reclaim themselves in two minutes, and the downloads come back on demand.
The same pattern applies to Podcasts and TV downloads and to Apple Music, and all three are worth checking in one pass since they behave identically.
Common questions
Where does the Books app store downloads on a Mac?
Inside containers under ~/Library, not in a Finder visible folder. That is why downloaded books and audiobooks are counted as System Data on the macOS storage screen with no way to drill into them there.
How do I delete a downloaded audiobook without losing it?
Right click the item in Books and choose Remove Download. The book stays in your library and can be downloaded again. Choosing Delete instead removes it from the library across your devices.
How much space do audiobooks use?
Around a gigabyte for a long title, compared with a megabyte or two for an ebook. A few audiobooks are worth clearing on a full Mac, while a large ebook library is rarely worth the effort.
Are PDFs in Books redownloadable?
No. Anything you imported yourself, including PDFs, exists only in your library. Copy those out before removing items in bulk, since they were never purchased and cannot be fetched again.