If booklets are little books, what are pamphlets?

Sadly, the term doesn't appear to be a diminutive of anything. According to this reference here...

The adverb pamphlet for a small work (opuscule) issued by itself without covers came into Middle English ca 1387 as pamphilet or panflet, generalized from a twelfth-century amatory comic poem with a old flavor, Pamphilus, seu de Amore ("Pamphilus: or, Concerning Love"), written in Latin. Pamphilus's name was derived from Greek, meaning "friend of everyone". The poem was popular and widely copied and circulated on its own, forming a slim codex.

Its modern connotations of a tract concerning a contemporary issue was a product of the heated arguments leading to the English Civil War; this sense appeared in 1642.

Flyer?

http://en.wikipedia.org/wiki/Flyer_(pamphlet)