Rather than post in his comments, i decided to post my short list (extremely short list) of some pieces that focus on timbre as a compositional method.
Basic ground rules were: Timbre had to be a major form of organization; one piece/album per composer (with one notable exception); give a nice cross-section of works both acoustic and electronic
some caveats: this is definitely a short list. it's missing all sorts of pieces by all sorts of composers. No noise musicians, no techno/house/ambient, not any "pop" at all really. not even the minimalists like Mikel Rouse (mainly cause I have no idea what I'd list of his...LOL). And, yes, there's a predilection of pieces by friends. But it's a thing we do, so why not?
Please add your own in the comments!
-From Me Flows What You Call Time- Toru Takemitsu
-Pente- Dennis Smalley
-Unsound Objects- Jonty Harrison
-Tongues of Fire- Trevor Wishart
-Inner Time II- Radulescu
-Les espaces acoustique- Grisey
-La Creation du Monde- Bernard Parmegiani
-De Natura Sonorum- Parmegiani
-2012 Stories- Paul Rudy (i think he's up to five or six discs in the series. "In Lake'ch" is the first, and quite powerful. He also performs with these live, and it's amazing.)
-Requien- Michel Chion
-Beneath the Forest Floor (off "Transformations")- Hildegarde Westerkamp
-Riverrun- Barry Truax (really need to listen to the 8 channel version)
-Time, Motion, and Memory- John Young
-Metastasis- Xenakis (though it was hard to choose just one!)
-I Am Sitting in a Room- Alvin Lucier
-The Light that Fills the World- John Luther Adams
-Ethers- Tristan Murail
-unhurried, untitled- David McIntire
-With my Eyes Shut- Jason Bolte (really, anything by Jason is amazing, but this piece in particular is astounding. Also, the it's in just intonation…makes those seemingly not too difficult clarinet lines tough…especially all those pesky held notes he put in there!)
-Bubolz Walk- Andrew Seager Cole (again, you can just sit on his site and listen away, but i'll go with his latest tape piece as an example)