So you have grown weary of trampling around your campaign world (be it a published one or one of your own invention) and would like to take the party somewhere really different? Then perhaps a jaunt into another plane of existence might be in order...
Chapter 1: Lore of the Planes gets straight down to business, starting with philosophical musings about what the planes actually are (as much as you can imagine then a whole lot more, apparently!). To make it a bit more comprehensible, think of them as tangible representations of concepts and ideas. The main ones are based on the alignments - things like Good and Evil, Law and Chaos - but you might find ones for Art or Music, Beauty or Trade... only they come and go as people find different ideas of importance. You also find the souls of those who have finished their mortal existence here, perhaps making their way to the Underworld or onwards to some final destination with a few devout ones being gathered in by the deity they venerated. And then there are the denizens of the planes themselves. Independent living beings who find their homes in these strange places. Perhaps this is where the Gods are to be found, complete with the minions and companions that their faith holds that they should have. Living vessels for the power of an idea. I don't profess to understand, there again if I did maybe I'd be a deity too!
Next, Chapter 2: Cosmology tries to explain what is contained in a sample cosmology, the Midgard one. Use it as is (even if you don't run your games in Midgard), adapt it or use it as a template and guide as you devise your own. The whole book is designed as a 'plug and play' manual, take the bits you want, add in your own ideas and come up with a set of planes like no other - it's probably as close to being a god as any of us will get! Like any religion, it starts "In the beginning..." How did the universe in which your campaign is run come into existence in the first place? And who found out and started to create legends about it (which may or may not be accurate, of course)? Maybe different groups have different explanations for how everything came to be - these lead to contention, be it academic debate or all-out war. Examples from Migard are given. Was order given to chaos, or the other way around? It's never static, that's for sure, and there is always contention between various aspects of the planes themselves, never mind mortal squabbling below. The Material Plane, the place where your campaign world exists, is at the middle. Denizens of myriad planes squabble over it because they all draw their power from the very souls of those who live there... and often meddle through dreams and visions or outright intervention in what is going on there, too.
And then you - or at least the party - think of going there. Most use magical means (a spell or portal) but some slip through the cracks into some kind of 'sea of possibilities' - maybe it presents itself as a corridor with lots of doors, or it might be something far more exotic. Through those doors (or via whatever metaphor you pick) are all these planes... and each plane has its own characteristics and nature. A selection of the Midgard planes are described here, for inspiration or use as you please. There are loads of ideas here, and many useful sidebars which show you how to use these traits and characteristics to affect game mechanics. In a Good-aligned plane, perhaps 'evil' magic doesn't work, at least, nothing more than a nasty smell or a bit of smoke results from your casting, for example. Or perhaps any spell-casting results in a bright flash of light in addition to the intended effects.
If that wasn't enough, Chapter 3: Other Locations looks at what else is out there besides the planes. The cracks between them, if you will. The places you might end up if you botch that planar travel spell or open the wrong door. Called Between, this unspace has a whole geography and inhabitants of its own and, trust me, you don't want to go there. Neither will your characters, if they know about it. They might be more comfortable in another unspace called The Casino, but beware: it's generally more than mere money that rides on the games played here. You can play - or bet on - just about anything conceivable here, and there's even a 'game development' complex where games from all over the known universes and beyond are tested and honed to a high level. There are other locations as well, if these two do not take your fancy: the Evermaw, the Marketplace, the Plane of Spears, and more. The Marketplace is an intersection of all the markets that ever there were, a place when literally anything is available - for a price. A multitude of adventures await in all these places, and if reading about them doesn't give you enough ideas, plenty of suggestions for how to use them in your game are sprinkled throughout.
Next is Chapter 4: Heroes of the Planes. So far, we have heard about assorted denizens of every plane discussed, but here you get the low-down on new races native to the planes along with new feats, traits, incantations and spells that may be learned here, may be useful here... or may be used against unwary visitors. Then on to the real heart of the matter with Chapter 5: Gamemastering Infinity. After reading thus far, you may be thinking that you have bitten off more than you can chew. Don't worry, there's plenty of helpful advice here. Start small. Add the odd twist to an otherwise-normal adventure. Remember that the planes never stay the same. Then there's an introduction to planar roads, the routes seasoned planes-wanderers use to get from one place to another. Even seasoned travellers find them tricky to navigate and often end up someplace other than they intended. Here also are the strange economics of the planes, the commodities valued here are different from the gold pieces that are useful back home on the mortal plane. This chapter ends with some magical items unique to the planes, and it is followed by the last chapter, Chapter 6: Bestiary. As you can imagine, some mighty strange beasties are to be found here.
This is a book of ideas, of inspiration and of concepts. Even if you stick to the exemplars pulled from the multiverse around Midgard, there is still much to be done before you can actually run much in the way of planar adventures... but this is a starting point to help you think about what you want and how to make it happen. It digs at the fundamental underpinnings of what makes a fantasy campaign world work, and what may lie beyond... but may be a bit philosophical for some tastes. An interesting read, nevertheless.
[4 of 5 Stars!]