Small-to-medium boxes, you might suggest.
What about something different? I've tucked some of my doll books away in a carry on luggage case. Yes, it's heavy, but I like being able to move a batch of books with one hand and a push.
There is no padding at the bottom of the case. I just added books and books.

I've managed to add some shorter, paperback books to the side.

To fill in the empty gap between the case top and the last book, I added two empty Mattel Barbie Fashion Fever tubes and stuffed a mosquito net to the side. Ta da! Book bounce reduction.

Now to zip close the case.

Neat, huh?