How would you want a plot to be presented?

[FlawlessHappiness]

(I don't think anyone reads the mementos)

Oh i do!

Remember mementos are something that is going to be there in a longer time, easily access-able and is short. I would prefer to look at my mementos than looking through all my notes to find the info i need.
I use mementos like: "The machinery is broken. I need to fix it somehow". This sets a goal for the player. No goal? No reason to continue.
Having a memento which is going to be there for a lot of levels, is a possibility. But don't make the goal to obvious. Don't say: "I need to get revenge on [PERSON]". More like: "Somebody got killed here!".