I'm in the same boat as you. I just started to buy them and am running into problems with NJO. All but 4 of the NJO books were released in paperback or were only Sci-Fi Book Club 2-in-1 or 3-in-1 Omnibus hard covers. The very rare exceptions to this are Traitor, Dark Journey, and The Final Prophecy. Those were only SFBK hard cover single book editions. Those are the ones I've seen sell for >$75 on ebay.
There are also issues with some of the Republic Era, Empire Era, and New Republic Era novels. Almost all of the 3 book series (Jedi Academy, Black Fleet Crisis, Bounty Hunter Wars, etc) were only released in 3-in-1 Omnibus hard covers. Otherwise you're stuck with only getting them in paperback. Some of those hard covers are easier to come by, but there are the exceptions too.
I have a fairly extensive and accurate list of all of the novels released. If you want, shoot me a PM and I can send it to you.