Stackless Scala with Free Monads is a fairly easy to read paper with many practical applications. It introduces trampolines as a means to avoid stackoverflow errors on programming languages of the JVM and generalizes them into the Free Monad, a structure which can help writing monadic expressions for any type constructor if a functor is defined. Free monads can be useful in tackling the expression problem which is part of another paper.
The paper can be found here:
and you are expected to have at least tried to read it when coming to the meetup so that we can efficiently discuss it.