Here are some examples:
* Booklet: A small book.
* Starlet: A young, aspiring actress.
* Piglet: A young pig.
* Riverlet: A small river.
* Earlet: A small earring.
However, "-let" can also have other meanings depending on the word:
* Applet: A small application.
* Ticket: A small piece of paper with information or authorization. (This is a rare case where "-let" is not related to size)
Overall, "-let" adds a sense of reduction, cuteness, or insignificance to a word.