The book can be summed up as a part-biography, part-analysis of the series. Not to be confused with the 'Concordance' books, this isn't a Dark Tower encyclopedia; more an intelligent and multi-angled exploration of the characters, the (book by book) events and the semantics behind Stephen King's epic.
The depth of Vincent's insight and understanding is illustrated by King himself, quoted on the cover of the book as saying it "opens doors to Roland's world that not even I knew existed".

Every aspect, every nook, every unfound door has been delved into here. Vincent looks not only at the series itself but at all the related books in King's universe. He finds significance in the most seemingly-insubstantial events, that we as readers would probably have passed by without a moments thought, and somehow seems to put together the pieces we never knew were there.
If you have any quarrels with the books that you need laid to rest (I won't say what my own were here, lest I spoil the story), and I'm sure we all have a few, you will almost certainly find an explanation or theory in here that will put you at ease, or even a whole different perspective to mull over.

What else is there to say? The book is definitely text-heavy, with only a handful of pictures lingering within its 350 page bulk. These few images show various places and landmarks mentioned in the books, and considering how fascinating these images are to look at, it's disappointing that there's not just a few more in there to break up the text. The book also contains a glossary of Mid-World speak, and the original poem 'Childe Rowland To The Dark Tower Came' by Robert Browning, in which the books are thematically grounded.
'The Road To The Dark Tower' is a refreshing and level-headed look at the world(s) of our favourite ka-tet, although for obvious reasons, it's best left until you've finished books 1 through 7.