:Well, as I told you, the sources I know of don't have much to say. It may be that there isn't much to say, of course, but given his hold over the caliph for two years, this would be surprising. If modern sources have little to say, then I suggest you search the primary ones and use the modern ones as complements. BTW, I've written a short article on the amir al-umara office. Feel free to add anything I may have missed! Cheers and good luck,