well actually let me tell u the truth. At least in the united states, there are a bunch of Chinese ppl working on IRT, some of them are really prominent. My advisor, Li Cai, from UCLA is for sure an outstanding one. I am saying that because u can try to connect with ppl, or even shot email to the authors of the books that u read. I am not sure if they do have time to reply and work on your specific question(s), but if you do it and ppl do have time for it, then u will get sth back anyway.