The DHARMA manuals were a series of binders found by Sayid in the Flame station in "Enter 77". Sayid looked at one about the food drop and then one about operations. The operations manual contained a map of the cable system and showed the location of the barracks. It also (most probably) contained plans of The Looking Glass.
DI 90M1654-21644 was the code that appears on the manuals. "DI" was likely an abbreviation of "DHARMA Initiative".
Food drop binder
DI 90M1654-21644 DHARMA INITIATIVE FOOD DROP PROTOCOL 960VKKS PL390
DI 90M1654-21644 DHARMA INITIATIVE OPERATIONS MANUAL 960VKKS PL390
The sides of the binders seemed to all be identical except that some were skinnier. They all had the Flame logo, the word DHARMA, and a DHARMA Initiative part number "DI 90M1654-21644".
Lostpedia user notinpdx has authored an imitation Dharm Initiative Operation Manual cover.
- A binder with a similar design to the ones Sayid found was included in the Season 5 DHARMA Initiative Orientation Kit. The codes on the front are 151VKKS and PL342.