I read most, including the entire second half, of Watership Down in one sitting. Granted, I was on a cross country plane trip, but the book was a lot more engrossing than I expected. Now yes, I had seen the movie already, so I knew not to expect a children's book (even though the original story was made up for his daughter). But the book goes into a LOT more detail about the rabbits' social structure, first describing their starting "normal" warren so that when they run into Cowslip's Warren, and then ultimately Efrafa, you can see how very out of the ordinary things have become, forcing Hazel and his Warren to adapt.
And I found it brilliantly believable. With the exception of Fiver's apparently psychic powers, these were rabbits acting like rabbits, forced into extraordinary situations, where they must view the world in ways they never had means or reason to before. And in the end, they come out better for it, because they accumulate knowledge, and they accumulate advantages. Because of everything that happens to them, they learn how to deal with the unknown, culminating in the final confrontation with General Woundwort and Efrafa.