Hi Johnny,
A commendable project - and something worthy of your amazing artistic talents!! I was recently considering doing my Masters degree in the history of the sling in the Mediterranean area (I have a degree in Ancient History), so would definately have an eye on that particular topic

BTW: I've written four books so far on obscure historical subjects (mainly cults, assassins and secret societies...!) under my RL name Paul Elliott.
I think Hondero is right - a shorter more concise book on the history would better be served by your illustrations and suit the market also.
That said, I would like to postulate a shorter, different index:
1.Introduction - What is the sling? Why was it used, etc? Use in prehistory. Evidence?
2.Biblical (Assyrian/Babylonian/Hebrew/Arab/Egyptian)
3.Greek & Rhodian etc. plus cestros
4.Roman & Balaeric
5.Byzantine slings & Staff Sling
6.Barbarian Slingers (Celtic, Dacian, German, etc)
7. Construction methods with modern recreations of slings and bullets. What about photos of modern re-enactors! I'm teetering on the edge of getting together a Greek slingers costume complete with high boots, sunhat, tunic, stone pouch and maybe chalmys (travelling cloak).