As such, this book is aimed at a subset of a subset of readers; not only must you be obsessed with the imaginarium of Tolkien, you must have a strange fascination with maps of arcane and obscure places. If you do not find yourself included in this exact set, I'd advise you to not waste your time on this review, and especially not on the book that it references.
For the two of you that are left: this Atlas is amazing. The author conducts an investigation of Middle-Earth and Tolkien's other created landscapes that is stunning; from the sheer volume of materials referenced and compared to each other, I don't think she could have done more research if she was writing a complete history of China.
The Lord of the Rings is full of frustrating hints at a large and complex backstory; every page begins a story, and then quickly moves on without ever finishing it. Karen Fornstad finishes all these stories, and probably manages to tell you how many people were involved, what language they spoke, the racial makeup of the group, and sometimes even the precise type of rock strata that their cities were built on (no, really.) The Atlas of Middle-Earth compounds and enhances the experience of reading the book it explores, and any Tolkien-o-phile has no right to call him/herself a fan unless they have a copy on their bookshelf.