What the diary contains, and why that matters
The diary is Don's personal record: names, accounts, and the operational details of a criminal network spread across several countries. Whoever holds it holds the architecture of the organisation — who works for whom, where the money moves, and which associates can be exposed or leveraged. That combination of intelligence and blackmail material makes it the most valuable single object in the story, worth more than any individual loyalty or any amount of cash.
For the police, the diary is the case they cannot otherwise make. They can watch Don, they can follow his couriers, but without documentary proof the network stays intact after any arrest. The entire police plan — recruiting a lookalike to move inside the organisation — is ultimately about getting close enough to find it. The impostor Vijay is inside the operation precisely because the diary is somewhere in it.
For Don's surviving lieutenants, the diary is both a threat and an asset. It can destroy them if it reaches the police; it can also re-establish whoever controls it as the effective head of the network. That tension inside the criminal organisation — who gets the book, who gets the power — runs alongside the main deception plot and gives the second half of each version much of its momentum.
How the two versions use it differently
In the 1978 film, the diary functions as a straightforward MacGuffin: its existence is established, it is sought, and its recovery by the right hands closes the plot in a way that feels morally tidy. The police get what they came for; the state wins.
The 2006 remake keeps the same object but uses it to set up the ending that changed. By the time the diary's location becomes clear, the audience has already been manoeuvred into a position where the revelation reframes everything they thought they understood about who has been running this search. The diary does not simply resolve the plot — it detonates it. What looked like the state recovering a criminal's records turns out to be something far more complicated, because the person who wanted those records most was not who the audience assumed.
This is the structural difference between the two treatments. In the original, the diary is evidence. In the remake, it is a secret about who the real investigator is — and that secret belongs to the film's final minutes rather than its investigation. The object stays the same. Its meaning inside the story reverses completely.