A fellow reader in Shelfari and Goodreads asked me to describe my research process for A Gift for the Sultan. My best answer to the question is in the video I've posted on my Amazon author page (also on my website and on YouTube). But here's a summary.
I sort of went crazy on the research; I found the period and the real characters so fascinating that I read everything I could find, in all the languages I read (only three, unfortunately — neither Greek nor Turkish). I think I was able to read almost everything relevant (in English, French or Spanish) because I focused narrowly on just a few months of history, from mid 1402 to end of 1403, when the nascent Ottoman Empire was suddenly smashed to pieces (for a short but critical period).
The subject was completely new to me — all my previous books had been about Latin America. But I had learned to do research. I joined an e-forum of scholars on Turkey and another one for Byzantinists, and I was so obsessive in searching through the New York Public Library and the very good collection in the New York University library that there came a point where those scholars were asking me questions. I even discovered a pencil-written manuscript in the NYPL archives, on a 15th century Turkish poetess, and passed that on to a literary journal in Ankara that used it for its inaugural issue. There were times when I thought my whole project was insane and I put it aside for months at a time, but the research was fun and put me in contact with lots of intelligent people. And I'm so stubborn that I kept working on the prose until I got it right, at least as far as I could tell.
I'm glad that this reader was interested enough to ask. Thanks!