Nate Teh Pwnzer:what happen to all the aspartan 2s and 1s? because i am to cheap to buy the books
Spartan-I project - as I have understood from reading bits and pieces of the novels and talking to my friend about it (who is a halo fanatic) the spartan-I project was to test out all of the technical/biological enhancements and armor that were to be used in the soon to come spartan-II project.
Spartan-IIs - The Spartan-II project came when the young spartan-IIs were found at around age 6 by Dr. Catherine Halsey, tested to be genetically suitable where she found about 150 candidates that were suitable then kidnapped about half (due to budget reasons) and replaced them with flash clones (fast growing versions of clones) which died fast of "natural causes" then taken to the UNSC military colony of "Reach" to undergo extensive and painful physical/technical modifications and trained in every aspect of military life/tactics. 23 of the original 68 spartan-IIs died from the modifications, 12 more became disabled from the modifications and were placed in high ranking officer positions since they couldn't handle the full rigors of combat, and the remaining 33 spartans became fully active spartan-IIs in the UNSC and given their armor and naval ranks. (they're not marines, which alot of people think since they fight along with marines all the time)
Spartan-IIIs - Spartan-IIIs were made as a cheaper, and slightly faster replacement for the spartan-IIs. (basically had very close to the same physical enhancements and used SPI armor instead of MJOLNIR) SPI armor seems to use some sort of active camouflage with weaker armor protection and no shield. only 3 companies were made, alpha, (which was wiped out due to a raid on the main covenent shipyard) bravo (which all got owned too except for 2 who now train spartan-IIIs) and gamma.
that's kind of a small description of the history of the spartan projects there's alot more info on wikipedia that I encourage u to check out...
http://en.wikipedia.org/wiki/SPARTAN-II_Project
oh, and yes, I've been told they're will be at least one other spartan in halo 3 :)