Books, Magazines etc. on Egyptian Philately

As Egypt was for many years part of the Ottoman Empire it takes its full place in this splendid book. Written in German with English and Turkish translations, it contains an Index giving the Roman and Arabic spelling of places with their map referencs, and includes all known Egyptian post offices up to 1914.
If it's revenues that you're interested in, then this is the book. It was published in 1982, and though more information has come to light in the past two decades (see, for instance, the meeting of the ESC 2000), these have yet to be published.
Kehr's original booklet on Interpostals, published in 1962. It gives types and place names for each, and has a price guide (optimistic at the time); Kehr notes that specimens with postal markings are worth much more, quoting at least $25 for an Interpostal properly used on cover.
Cockrill's update and reworking of Kehr in the 1980s. There remains a certain amount of dispute about the conclusions drawn by Cockrill, but he does suggest that covers with Interpostals should be worth at least 100. So $25 in the Sixties becomes 100 20 years later ... you do the maths and see the rate of return if you want to justify your "investment" in these unusual circular "stamps".