Винни-Пух

It was the first book I've read in English, meaning the unadapted text from the original source. And despite me being 26, I do recommend this book no matter how old you are. It was such a pleasure to read it! The drawings of Ernest H. Shepard are absolutely adorable and help to invoke the atmosphere of childhood dreams, when every moment of existence is full of joy and an inexpressible vague presentiment of miracle.
I'm sure there's not a single person who hasn't read this book in his or her mother tongue. However, reading it again in a foreign language wasn't boring for me at all, even though I knew the main plot. In my opinion, actually it is one of the best choices to start reading in English. I'd never done it before, so I really had some kind of psychological barrier. I wasn't confident and even had some fears. So I decided to pick something light and easy to understand, but meaningful at the same time. I didn't want it to be one of those 100 % childish fairytales.
As for the practical benefits, I learned some new words from the text. For instance, "to trot", "cork" etc. So I'm sure if your language level is intermediate (which is just the right moment to start reading unadapted texts), you'll find something new for yourself and besides, the process of reading will bring you real joy. Good luck!