All the time!
Sonar is a terrific Sound Design tool! Now I'm not using it to create libraries, but rather sounds for live theatre, so my toolbox may differ from yours.
First you need source material - I have about 1TB of sound effects libraries that I've used for a very long time, so they tend to be my primary sources. I do have a number of Kontakt libraries that I've started to use, a bunch of percussion libraries from SoundIron and 8Dio, Heaviosity Evolve, and a bunch of others I can't think of at the moment. I also record a lot of my own sounds.
Second, you need Effects plug-ins - all the SoundToys plugs are useful, and Waves has several that work well for sound design (e.g. MondoMod). I'm a big fan of delays for sound design, and the PSP stuff is awesome. There are probably a couple others I use, but again can't think of them at the moment.
Third, you will want a synthesizer or two for sweeps, swooshes, etc. The included plugins will get you started, then you'll want to investigate things like Alchemy or Diva. If you have the NI collection Reaktor is amazingly deep too.
You'll eventually want to try your hand at granular synthesis, there is a free processor called PaulStretch which will keep you occupied for years.
Lastly, you may find that a good audio editor still comes in handy. You can do a lot (A LOT) with slip editing in Soanr, but every once in a while I find it easier to think in terms of the old days (think tape) and I use Sound Forge and Wavelab to do some stuff. (In truth, sometimes I still use my tape deck... but I need to get over that!)
Tips -
listen to a LOT of stuff you want to emulate. For me it is sound tracks, but it could be anything sound design oriented, games, electronic based music, etc. Just LISTEN!
There are a handful of books that you might find useful too. I just got Burt's book about creating the sounds for Starwars and it will really open your mind to using sounds in less than conventional ways. If you can find the interviews with the designers for "Twister" they are eye openers too.
All of which leads up to creating a library. I don't do that, except for personal use, so my best advice would be to pick a sample engine (Kontakt, Mach5, SFZ) and learn as much as you can about the engine. If you want to learn to create Kontakt libraries I'd suggest joining VI-Control, a forum that boasts a number of the top Kontakt developers. Not sure where to look for the others.