We've got a bunch of those floppy square picture books - the kind on the racks at B&N that you can buy for under $5. The thing is, they don't stand up very well on a bookshelf - they just slide all over and because they are floppy, it is more difficult for the young 'uns to get them back in the bookshelf too. Similarly, we also have a bunch of those early reader books - taller and thinner, but just as floppy. So, I've found that a great remedy is to use magazine holders to keep them all neatly in one place. The kids will always know where to put them away and it makes it easy to take them and go as well if you just pull out the whole holder.
You can spend a little bit to buy sturdy ones that will look cute and last a long time - otherwise you can do as I did and pick up a package of 5 white cardboard fold-it-yourself holders at IKEA for under $3 and tape the bottoms with packaging tape to keep them from unfolding then decorate them yourself if you like.