IBooks removable?

They can be deleted from the device. But you should keep a copy on your computer with iTunes in case you want to reinstall the book.

You can delete a downloaded copy of a book via the Select button at the top right of the bookshelf.

If the books remains in the store, and if you don't move countries (or hide it from the cloud : Hide and unhide iTunes Store, App Store, or iBooks Store purchases - Apple Support), it will then show in the Purchased tab in the app for redownloading.

But in case the publisher/rights-holder removes it, or you move countries, before deleting it you can copy it over to your computer's iTunes library by connecting the phone to it and doing File > Devices > Transfer Purchases (and if using a Mac with iBooks on it then do File > Move Books From iTunes in its iBooks app).