The Planes were a common high-level adventure region for Dungeons and Dragons for several reasons. Tough opponents could exist, and the environments themselves could be fantastic and dangerous. This book covers the 'Great Wheel' or a detailed model of how various other places connect with the 'Prime Material Plane' or 'normal' world.
From a technical standpoint, this is a good-quality scan that is legible, clear, and has a good selection of features. Text is searchable, there's bookmarks for every section, etc.
As to content, this is a relatively ancient book, and it does show. This is not a 'reader' for all but the brave, as it's very dry and the text is somewhat repetitive, with each plane discussed receiving a standard format writeup containing how to survive there, random encounters, special places, etc. It's very dry, and definitely organized as a 'reference' int he sense that if a DM is planning a trip for his characters to The Abyss, there's a few pages covering all you need to know to make survival an interesting challenge.
[4 of 5 Stars!]