Whenever I have information that needs to be revealed I have a sliding scale. Highest level of success gives you a direct easy to follow answer that let's the PCs know one or more route to success along with relative risks and rewards. Lowest level of success may give a clue to where to look for further information or a high risk low reward option depending on the situation.
The highest level of planning is generally due to careful planning, potentially along with luck (although I try to avoid a single die roll). Some examples of what I might try
- Try to infiltrate the BBEG's organization. This might involve a lot of work and planning, and my not suit the group.
- Find an ex member of the BBEG's organization and convince them to cooperate.
- Through research or rumors, they find out info about one of the lieutenants. Maybe they have a bad drug habit and sneak out to get their supply, maybe they're unhappy with the BBEG. That opens up a couple other possibilities
Trick them Mission Impossible style. Make them believe something is true at isn't using disguises, illusion and other magic. Maybe lead them to think the group will help them overthrow the BBEG and put this guy at the top.
Kidnap them and try to get them to talk. I don't allow torture, so there still has to be motivation for the lieutenant to give info. Withhold the drugs, threaten to expose them and so on.
- Have an informant that may or may not be totally on the up-and-up
Anyway, so those are some of my ideas to get info to PCs.
But that doesn't answer your question directly, just other avenues for thought. So there's always the ubiquitous diary which admittedly I do use sometimes. But there are other options as well.
The eavesdropping servant. Too often the only denizens of the BBEG's lair are the level appropriate warriors. Sometimes that make sense, but frequently I have to ask - who cooks the food? Is Brog the Destroyer going to do his own dishes? Maybe, maybe not. So instead of getting info from the BBEG, get it from his steward who is always within earshot just in case his master calls.
The evil guy monologue. Also a cliche', but bad guys like to taunt their enemies, before during and even after the fight. The bad guy may be bleeding out, but still believes in the cause and gloats, letting something slip unintentionally.
Paperwork. Not nearly as exciting as the diary, but even evil organizations need the proper forms filled out. Maybe researching the billing paperwork leads to a supplier who could give leads.
The message. Big bad just died? Well the carrier pigeon flies in, the courier knocks on the door or the sending stone pings with details that leads to the next clue. Includes correspondences that were previously received but not destroyed.
Of course there's always the diary or logs, and there's nothing wrong with that. Can make for some nice handouts.
So those are my 2 coppers in addition some other good ideas.