outlet, booklet, tablet... Are there any other word that contains a conjugation of "let" with other words? Why do these words contain let? If not coincidental, what is the significant meaning of let in these words?
Even though all three words you've mentioned contain let, that doesn't mean that they have something in common.
- outlet comes from two words: out and the verb let (see this Google search).
- booklet comes from two things: the noun book and the suffix -let that means small, lesser and comes from Old French (see this page for proof).
- tablet comes from two words: the noun table and the Old French word tablete (again, check out this Google search).
So, the three words all having let at the end is just a coincidence.