I agree with wintertime. A point and click adventure game shouldn't be any different than the casual "find the hidden object" games that have been so successful in the last few years.
This type of game is a picture on a screen, with various items in the picture producing some reaction when clicked. That's it. The "game" elements come in by connecting various pictures (the scenes of the game) to each other and a story to guide the player from one to the next. Plus some "triggers" to activate some objects in pictures or cause certain objects to have different effects.
I think you may be overthinking this. I don't know of any open source software, books, or guides tailored to making this kind of game. The golden era of adventure games was in a time when only professional studios would be making them. Today, they are minimally popular (though I still like them) and also not very complex as compared with other software. These two factors make it unlikely that there's much out there like what you're looking for, but the fact that they aren't so complicated also means that you shouldn't need them too much.
For an exercise, try this:
Have a screen with a blue background and a few red squares in it. Define events for the squares so that when they're clicked they change to a different color. Have one square cause a transition to a new screen, with a green background and blue squares and a similar click-and-change design.
Once you've done that, you've demonstrated all the basic skills you need to make this kind of game. Seriously. The rest (from a programming perspective) is basic game loop kind of stuff, which you say you've done so that shouldn't be an issue. The rest (from a polished game perspective) is better art and a story that determines the screens a player can visit, and under what circumstances.