Another thing you might consider for transferring books to an Ipad is the Sony PWS. You can put a large micro SD card in it and install the app in your Ipad and easily transfer books wirelessly from the device. It's twice the size of a thumb drive and it has it's own Wifi. The app is available for IOS.
I'm an Android user so I'm not sure how the PWS app integrates with IOS but I'm sure you'll be able to find out about that easily enough. These devices are cheap and reliable. Here's a link on Amazon:
https://www.amazon.com/Sony-Portable...words=sony+pws
That's for the USA Amazon site but that's the one I have the link handy for. It'll give you the information about the device. There are also a number of good reviews of them on Youtube. I've been using these with Android for years with never a problem. I helped someone set his up on his Iphone last week so I know it works okay but that was only about videos. I never looked beyond that with IOS.
Also there are a lot of newer devices of this type. You probably want to look around if you do think of going this way. I pointed to this one because I know it's reliable and cheap. I've used the Sandisk Connect and the Mobilelite as well. They cost a lot more but work as well.
Barry
Barry