If you have a lot of books, then may I suggest putting them into a cloud storage such as Dropbox? Using Dropbox as the example - you get 2GB of storage space, which is a lot for e-books.
Once the book are in Dropbox, you can access them via the [free] Dropbox app on the iPad. Then, using the "Open In" function, you can choose to open a book in the Kindle app.
One benefit of this is that you are not putting all the books onto your iPad at once. So, you can save a bit of storage space and let them stay in the cloud storage until you are ready to use them.
I find the Dropbox option easier, mainly cause putting the books in Dropbox all at once is faster than individually sending the books (whether by e-mail or Send to Kindle). And, I only put the book I want to read at that time on my device.
Maybe this will work for you?
Marilyn
P.S. It doesn't have to be Dropbox, of course, as the cloud storage. Just any service that you can use to access the books from your iPad. Dropbox is just the one I use...