House Rules for the Manuals of Improvement

I have always house-ruled the following for all of the Manuals:

1. Once read, the Manual disappears into its own pocket dimension and does not reappear until the 100 years passes. This reappearance happens in a random library or bookstore in the Prime Material Plane.
2. Each Manual can only be used once per character.
3. All of the Manuals have protection against detection through magical means. Even Detect Magic doesn't detect them.
4. The Manuals are removed from the random loot tables. They are specific rewards that I purposefully put in the game for the players to find.

I've always found that players rarely get upset at house-rules, as long as they are discussed, make sense, and are consistent. Don't be afraid to say, "Hey, I've been thinking about this and doing some research and here's my ruling moving forward..." at the next session, even if that contradicts what your ruling in the moment was in the last session. Just avoid retconning unless it is necessary.

Just remember that your rules should also apply to the NPCs and enemies. Don't say they can't do something, and then let one of your BBEGs do exactly that.

House Rules for the Manuals of Improvement

I have always house-ruled the following for all of the Manuals:

1. Once read, the Manual disappears into its own pocket dimension and does not reappear until the 100 years passes. This reappearance happens in a random library or bookstore in the Prime Material Plane.
2. Each Manual can only be used once per character.
3. All of the Manuals have protection against detection through magical means. Even Detect Magic doesn't detect them.
4. The Manuals are removed from the random loot tables. They are specific rewards that I purposefully put in the game for the players to find.

These rules keep the players from abusing the Manuals, and I have never had an issue with the players understanding why I use them. I try to make house-rules only when something in the game is too ambiguous or if I feel something is unbalanced. In this case, it is specifically because I feel situations like you are seeing are possible and I wanted to cut off any abuse.

I've always found that players rarely get upset at house-rules, as long as they are discussed, make sense, and are consistent. Don't be afraid to say, "Hey, I've been thinking about this and doing some research and here's my ruling moving forward..." at the next session, even if that contradicts what your ruling in the moment was in the last session. Just avoid retconning unless it is necessary.

Just remember that your rules should also apply to the NPCs and enemies. Don't say they can't do something, and then let one of your BBEGs do exactly that.