Frog and Toad are Friends

This is an excellent first novel for young readers by Arnold Lobel. It has been used for a grade 2 or 3 reading level. In the story it tells of a friendship between a frog and a toad and the adventures and misadventures between the two friends.
What is unique about this story is that it can be used as a center piece of a unit on frogs and toads, which is part of the great 2 and 3 curriculum. One can explore real frogs and real toads, their habitat, and their lifecycle. At the same time this story explores the ideas surrounding what makes a good friend and why friendship is important.
Although I have not read the entire series, I am aware there is a number of different Frog and Toad books by Arnold Lobel. Has anyone else read any of the other books and care to share their thoughts and feelings on any of the books. Please feel free to share.