For non-PDF books, I believe they are DRM protected so they’re not going to work anywhere else. It’s a bit of a bummer but the book publishing industry is probably even less progressive than the music industry since other platforms have this same restriction unless you specifically loaded a DRM free book onto the device (Apple Books supports this).
Funny enough, some PDF books are similar, although it’s not mandatory Apple support this function but in the PDF there is a parameter to say if the PDF can be shared or not and Apple Books (iBooks app) uses this, you will notice when you press the share tab on a PDF some have the email option and others don’t, took me a while to find out what was going on. This is quite a frustrating choice by apple, probably to appese the publishing industry to get better rates. I have some fairly innocent broahures and mapping PDFs in iBooks which I can’t email because the person who generates the PDF set the parameter, probably unknowingly !
The only good thing about Apple Books now is you can load PDFs and DRM free (external sourced) books into the app without iTunes anymore and this synchronises to your other devices. I remembered when you had to use iTunes or email to get PDFs onto an iPad and have subsequent versioning issues.
