First, be careful you don't buy the TTT or RotK RULEBOOK. I haven't seen them myself (having come to the hobby after the OR rulebook was published), but from my understanding these things are still floating around on eBay and such. They are out dated.
That being said, the Journey Book for each of the LotR Book/Movies is a good investment in my opinion. There are some profiles in each which you can find elsewhere, but the scenarios included in each allow you to play out most of the key events for the trilogy. Getting the models for each is a goal in and of itself of course.
There are also nice painting and terrain making guides in each which could help you along depending on your background and experience.
Beyond those three, the others are best targeted to your areas of interest. The Mordor and Harad books are very good for developing armies from either of those regions (or to give you good info you play against them regularly), but won't help you much if you want to build an Elf or Gondor force for example. So, as mentioned earlier, if you want to post some of your interests regarding armies we can help direct your money most efficiently.
As a side note, most of the supplements really do have a lot of interesting information and can be very helpful in inspiring you in areas you didn't initially consider. I do suggest that you keep an eye out for eBay or local sales of them and snatch up ones you find at a good cut.