It's basically a HP encyclopedia; every piece of factual information from books 1-5 has been compiled into various sections (Ministry of Magic, Hogwarts, etc). And I mean EVERY detail. Every name. Every book. Every spell. All the backstory is pulled together. There are maps of Hogwarts and Hogsmeade, plans of the Minstry and the Dursleys' house, a VERY comprehensive timeline, family trees, etc, etc. It's exactly the type of book that I wished I'd had as I read through the HP books and made pages of scribbled notes. It's not a substitute for the books, but exactly what it says it is - a "companion".

Now the caveats.
Some of the detail in here is not strictly "canon" - there are many assumptions and "logical extrapolations". Just because you read it in here, don't assume that it has been explictly stated in the HP books.
The design is awful. I know that this is mainly because it's a small-press (or even self-published?) work, but even so...the cover is beyond awful and will do the book a great disservice. The internal illustrations are childish. The plans and maps look like they have been drawn by hand on graph paper. There is not enough spacing in the text - everything looks cramped; the density of the text will certainly be off-putting to younger children. This all seems a little strange, because the proofreading and editing seem to be fine. I can't believe a bigger publisher didn't pick this work up.
With Book 6 out, some of the detail is obviously wrong/out of date. This could obviously be corrected with a later edition - hopefully with a bigger publisher.

So...buy this book. But cover it with brown paper to disguise the monstrosity of a front cover picture. Please don't let the picture on this page put you off!