My take on things:
- The campaign was missing too many details.
Generally, I'd say you need to flesh out the big picture first. What's happening in the world, who's fighting who, etc. Then work your way down from that. Don't go overboard on the details though; sometimes the details just distract from the story. Besides, you should be able to make up some details as you; just write down the details for future reference.
- I was unorganized.
Before each session, take your time to prepare/update the next two or three sessions. Fill in some of the missing details, including some details about the area the PC's are currently moving through.
In my gaming group, after each session, one player has to write down what happened, as their character would do it (different player every session). At the beginning of each session, they have to read the story to the rest of the group. If it's a good story, that player gets some extra XP.
We also have a short quiz at the beginning, where the DM asks about specific details (names of NPC's, etc) for extra XP. This keeps the players involved, and lets them organize for you.
- I didn't have a firm enough grasp of the rules/system.
In my group, the DM doesn't know everything either. We can look up details if need be, or he can use common sense to resolve the situation. Just try to know as much as possible; in D&D that's the best you can do.
- Players became distracted--they joked around, and easily became unfocussed.
This can happen because the story isn't very compelling, or because they're just not that interested. Or maybe they have other things on their minds, or just feel like joking around. It happens in every gaming group. Talk to the people involved; they should be able to tell you *why* they joke around...
- Unsure of what to do regarding players who couldn't make it to a session, which, as adults, happens often enough.
We generally leave the character out of the session. He or she is asleep/sick. This isn't very realistic, but it works for us. It also adds some spice to the session: your best fighter isn't there, so how are the remaining players going to take on those monsters?
- Overall, the players may not have been invested enough in the campaign.
Let them describe their character's background. Award good roleplaying, instead of just beating monsters. In general, try to create the story together; you provide the background, they fill in much of the detail.
- As for the players reading "forbidden" books: they may know what a mind-flayer is and how to beat them, but their characters don't. If they abuse the knowledge, give them less (or no) XP. If a player insists his character knows certain details, have him/her explain why. There are stories and legends about monsters; perhaps he/she heard some of those. If it's a good excuse, is realistic, and fits the character, go with it. You can let them do a WIS or INT check for it too. Again, reward good roleplaying.
If a player keeps bringing up forbidden knowledge, you could use it to add some spice to the character. He may be an aspiring scholar or sage; at every oppertunity, he should go to libraries to expand his knowledge. Or he always seeks out experienced NPC's to ask them about their battles with exotic creatures. Have the player write down the information and the source. When a situation involving forbidden knowledge comes up, he can check his notes. If it's on his list, he knows it (or has access to it in his books), if it's not, he doesn't know it (or has to do an absurd WIS/INT check).