If you want to know the origin of this idiom, read it on this website: http://www.phrases.org.uk/meanings/raining%20cats%20and%20dogs.html
We give books is a website created mostly for kids, nevertheless it gives great resources to adult learners too.
You must register, but it’s free and it’s worth it!
For example, if you are struggling trying to learn weather vocabulary, this book could be helpful:
“Weather” by DK Publishing.
This is the website’s link: http://www.wegivebooks.org/
“We give books” supports worldwide charities. You can pick an organization and each time you read a book you will donate to that charity the book you’re reading.