The Changing Land is several leagues of territory beset by waves of transformation. Anything can happen there, at a moment's notice: volcanoes, acid pits, monsters sprouting from the ground, toxic magical winds. Pretty much everything that happens can kill you. Sorcerers from all over the world are trying to cross to the Castle at the center, on the theory that there's gotta be something good there. (If nothing else, an off-switch.)
Into this mess comes Dilvish, who you may remember from some short stories. He's still hunting his arch-enemy Jelerak. (The Castle in the Changing Land used to be Jelerak's castle.) All Dilvish has to do is cross miles of deathtrap territory, contend with any of Jelerak's servants who remain plus every other sorcerer as lucky as he is, and find the evil wizard. Dilvish is equipped with an iron horse and Boots of Elvenkind (Dungeon Master's Guide, p139). The horse may be the smarter of the two.
I should try to explain what it was about Zelazny, back in those early days of genre fantasy. His characters are... modern without being contemporary. The sorcerers in this book don't know what a grandfather clock is (the Castle is full of anachronisms), but they complain about office politics and being dragged out of bed when you crystal-ball them in the middle of the night. They have girlfriends. (Sometimes the girlfriends are smarter than they are, too.)
(Yes, there are girl sorcerers too. That's why the Brotherhood of Sorcerers is now officially called the Society, and don't you get it wrong, or a bunch of sorceresses, enchantresses, and wizardresses are going to land on your ass. The question is assumed to be settled. I said it was modern, but modern 80s, ok?)
There's also a better hacking scene than cyberpunk ever managed in that entire decade. Spell-hacking. It is absolutely recognizable to any programmer-type person. (This book came out the same year as Vinge's "True Names". SF about computers always dates itself by trying to be current with the future, but the spell-hacking is a metaphor and therefore timeless. It's genius. Someone needs to write a damn monograph about it.)
The story manages to be epic in scope while being homey and comfortable. Everyone is parading around in a castle older than time -- I didn't say Jelerak was the first owner -- but it's not about grand battles and charges of glory. More like a comic melodrama, with escapes and schemes and sneaking about. The (self-called) light wizards and dark wizards would rather talk practicalities, and even the villains manage to not do anything very horrible on-screen. Much.
The scenery is as wildly imaginative as fantasy has ever seen. Zelazny can just riff ideas forever. You may be used to it from Amber's shadowride scenes, but the Changing Land doesn't go by in the rear-view mirror. The characters are stuck in it and have to deal with it, demon or spell or volcano or rain of singing frogs, whatever it is.
And in all this genre-trope mummery, Zelazny still feels free to occasionally turn loose his narrative voice, unconstrainable and hilarious as starlings bursting up into the sky.
I don't think I can overstate how much of an influence this book has had on me. Oh, there are plenty of Zelanzy books, and plenty of magical-infinite-house books too, and I took something from all of this. This may have been my first of each, though.