As we learned in "The Constant," the Black Rock ledger was the journal of the first mate of the Black Rock ship.
According to the auctioneer (and other sources like find815.com), the ledger is the only surviving artifact of the final voyage of the Black Rock, which was thought to be lost at sea following its departure from Portsmouth, England, March 22, 1845 on a purported trading mission to the Kingdom of Siam. The ledger was discovered in 1852 among the artifacts of pirates on Île Sainte-Marie, an island off the coast of Madagascar that at the time was a notorious pirate haven. It eventually came to be owned by Tovard Hanso.
The contents of the ledger were never made public and were unknown to anyone outside the Hanso family.
So what in the world is this about? I think it can be easily surmised that Widmore wants the ledger to help him learn about/locate the island.We know that Widmore was certainly working alongside the Hanso Foundation, but in what capacity is still a mystery. Here's a possible history that I've come up with:
Back at the turn of the century, the Black Rock runs aground at the island. Magnus Hanso discovers a lost civilization much akin to Shangri-La. Magnus (or the first mate) leaves the island with the ledger, telling people of the amazing discovery they made. The Hanso family, with perhaps the wrong intentions, makes a difficult journey back to the island.
Chaos ensues. Like many islands that were colonized by westerners, the indigenous population is killed off and the island is mined for its resources. After decades of having the island to themselves, the Hanso family sets up the Dharma Initiative, to allegedly solve the problems of humanity (or maybe just finding new ways to manipulate and profit from the island).
Some members of the Dharma initiative learn about the history and magic of the island, decide to revolt against the Hanso foundation, and create a new utopia/Shangri-La of their own. Ben gains power within this group, and they take over the island and all of its communications, thereby making it once again invisible to the outside world.
Widmore, whom has been working with Hanso and profiting from the discoveries made on the island, decides to take matters into his own hands. Now that communications are controlled by the insurgents, he acquires the ledger, which is the only known guide to locating the island. He holds a boat race around the world, perhaps in order to get people to end up there. He knows that flight 815 crashed there, and in order to stop people from searching for the wreckage he plants a fake plane at the bottom of the Sunda Trench and backhandedly gets Sam Thomas to find it.
Penny Widmore knows something about what her father is up to, and when Desmond goes missing she learns that her father sent Desmond to the island. She then sets up her own team of people to search for the island. When the hatch implodes, she is able to locate the island.
But Charles Widmore is also able to locate the island, and sends out a boat to go there. He hires Abaddon to be in charge, with the main goal to take Ben out in order to regain control of the island and its communications.
But for whatever reason, it doesn't work...
In the future, control of the island is still in Ben's hands. Abaddon is still on the hunt for him. Sayid, after trusting Widmore and then seeing one of the losties killed because of it, sides with Ben in order to protect the rest of the people on the island. He starts offing key people in the Widmore Corporation, but aims for Charles Widmore himself, "The Economist."
