Our very, very favorite book to read when it rains is The Rain Came Down by David Shannon. It's such a fun book to read aloud that I don't even mind reading it over and over and over. First of all, the kids get quite a kick out of me making the animal sounds instead of just reading them. Before you think this is another farm book (it's not), what we have in this story is a comedy of errors, all to be "blamed" on the rain. Think of falling dominoes . . . one mishap leads to the next. With all best rains, there's evidence of a rainbow at the end. Rush to your library and read this to your children, even if it's not raining!
In addition, our very, very favorite poem to say when it rains:
by Robert Louis Stevenson
The rain is raining all around,
It falls on field and tree,
It rains on the umbrellas here,
And on the ships at sea.