any good reference books on Itosu?

I have become interested in Itosu since I took a look at Pinan shodan using the Maxims of his students, his precepts, and clinical data. Now that I am trying to see who the Itosu was I cannot find much about him beyond what I have used. There are some books that point out only years and list his students. Some are extremely questionable written by people who did not know him nor did they have any references for their ideas. Currently, I am using the dates and trying to use history books to fill in the gaps. One speculation that I have been able to work out is that he might have been educated in China as a child. Again this is only based on combining some writings with about his skills and position and putting it together with history and how the RyuKyu kingdom would educate those who would get that position. I am almost losing it as it seems that most is mere speculation. I usually work in Anatomy, Bio mechanics, and Biology where things are easier to put together so any help will be appreciated