I wanted a book that I knew almost by heart. This is the best way of learning a language by reading: gathering what words mean from context, rather than having to look them up in dictionaries all the time. When you know Bilbo is saying one thing, you can make sense of the words more easily.

Also, it is a long book in much the same style throughout, which makes it possible to get used to the rhythm of it and get past that to the language.

I'd say it is a very good choice for learning a language.

Edit: that said, for French I think I would have chosen Dumas. In fact, I did.