Autechre have gone through some pretty distinct phases. I'll stick to the albums...
'Incunabula' and 'Amber' (maybe along with Aphex's 'Selected Ambient Works Vol 1') are basically the blueprint for the mellow end of IDM... artists like Phonem, Arovane, Xela, Opiate etc. Very synth based, a lot of drum machine beats, etc.
Around 'Tri Repetae' and 'Chiastic Slide' the sounds start to get more industrial, touches of distortion, hissing noises, the infamous sleeve note "all surface noise is intentional", etc. and the beats get more fiddly. Melancholy synths are still very much the order of the day, but things are getting a
little more difficult. A clear influence on groups like Funkstorung, Funckarma, a lot of stuff on labels like Merck, MD, and so on.
From the 5th album (untitled, but usually referred to as 'LP5') things get more process-oriented. It's a completely subjective call, but for me the music gets more and more cold and less "human". The rhythms get more complex, the sounds more noisy and they go nuts on all kinds of techniques used in electroacoustic music. To my mind they cross the line from pop/folk music to academic music, although not so much in their remixes and on their last album, the often mis-typed 'Untilted'. I mostly hear people enthusing about 'Confield' out of their more recent releases.
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As you may be able to guess, my interest has waned. I went fucking nuts for them in the early/mid 90s, they were basically my idols and I listened to their albums over and over... but my favourite period is the middle one. If you can find 'Tri Repetae ++' you get the album + the 2 EPs that came out before it, 'Garbage' (4 amazing close-to-ambient tracks) and 'Anvil Vapre' (where they introduced the more industrial sounds for the first time, worth it for the mighty opener,
Second Bad Vilbel).
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I'd second the recommendation of Aphex's SAW I over RDJ album, but it depends what you want. If you want skittery drill-n-bass beats mixed with whimsical melodies, then obviously go with RDJ. I think SAW I is just ... a fucking classic. And I don't like Aphex much at all. Beautiful, kinda naive tracks and such a stark contrast to today's electronica... shitty sounding tape recordings, lots of hiss and fuck all clarity, synths overdriving accidentally, beautiful beautiful atmospheres...
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Listening?
You can actually come across one or two example tracks for a lot of artists on Youtube or Google Video or wherever. Also, Warp's MP3 shop, bleep.com has heeeeappppps of great IDM music on offer and you can listen to decent length previews of almost everything there. So even if you don't want to use the shop, that's a good way to hear the music.
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Recommendations will be kind of random, bceause there's not much common ground between the artists you mentioned in your first post, but here are some: (labels in brackets)
Two Lone Swordsmen - Stay Down (Warp)
Kim Hiorthoy - Hei (Smalltown Supersound)
The Remote Viewer - Here I Go Again On My Own (City Centre Offices)
various - Intermission (Plug Research)
The Black Dog - Spanners (Warp)
The Bionaut - Lush Life Electronica (Harvest / EMI ... I think)
Mouse On Mars - Iaora Tahiti (Too Pure)
Aspen - Are You That Retail Snob? (Involve / Surgery)
Electric Birds - Strata Frames (U-Cover)
To Rococo Rot - The Amateur View (City Slang)
There's heaps of great singles etc. out there, but a lot of it is lost in the mists of time (although probably coming back to an MP3 blog near you

) so I've tried to give you a list of things that I believe are still in print.