Hi izzig
Although I too have found books that say melissa is a middle note, I would suggest that melissa is a top note as its main chemical constituent is citral a Monoterpenoid aldehyde which from a chemical point of view together with esters suggests a top note. The oil its self is sharp, penetrating and volatile which again are common characterstics of a top note. In the plant top notes are found in flowers, leaves and fruits. Melissa in my opinion wouldn't be suitable to use as a body of a blend as in a middle note as they are usually warm, round, soft and mellow (not like melissa) also middle notes usually form the bulk of a blend between 40-80% (not sure I would like to add this much melissa to any blend).
Hope this helps and doesn't confuse you more
Love and light