Ebook: An Introduction to Middle English (Edinburgh Textbooks on the English Language)

If you just want some guidance on reading Chaucer and other ME authors, this book is probably not the place to start. But if you want more basic information on the development of the language, focused on ME, from a linguistics approach, this is a very good resource. I had no formal training in linguistics, but enjoyed acquiring some of the basic concepts through this book. Although its treatment of ME might be rather "formal," I found it accessible and appropriate for my level and interests, which admittedly might be more "academic" than the average reader. It's a book I keep coming back to.
Check out some of the preview pages offered on Amazon to see if its what you are looking for. I haven't found anything yet that treats the material covered as lucidly and succinctly--but what it offers might not be what you are looking for.