It would be nice if after every 'block' of learning, you get an advice on what books to read, a short list of books that are suitable for the level that you've achieved. No problem if these are kids books in the beginning... Reading helps you learn a language!
The saddest thing is that there are almost no books, even kids books, written in present tense only; so, unless you learn grammar somewhere outside Duolingo, you won't be exactly ready to read books until about two-thirds down the tree. It will probably make more sense to read adapted texts from textbooks or special readers at the beginner level. It would also be great if there were such adapted texts right on Duolingo.
To add something concrete to these general thoughts: For French I can recommend "Le petit prince". It was the first book I read in French (of course I had read it before in my native language) and it was a nice start for me. However, to understand all the grammar structures used there I had to learn Passé Simple which is not taught on Duolingo at all as far as I remember.
I ran across a web site which has e-books that are in the public domain. It supports several languages: English,Spanish, German, French, Italian. I have not yet used it but it looks promising. http://www.feedbooks.com/books/top?lang=es I have looked at the bilinual children books at the public library, but as olimo said, the grammar is not on a beginner level with a small vocabulary. But I am sure it is worth a try. Some libraries can order from bigger libraries on loan. I think I might just try that myself. I try to find free stuff, because once you finish with the item and move on, you are stuck with it.