No, it's not a typo -- parentheses is the plural of parenthesis. The first rule of using them is that there should always, always be two, but thanks to their natural shape it shouldn't be too hard to keep track of that. The rest of the rules of using parentheses may not be so easy to remember.
When Are They Used?
The parenthesis doesn't appear a whole lot in fiction writing, but it can be an effective tool when it's added rarely. In fiction, parentheses most often indicate an aside, or an extra bit of information, that the author is giving to the reader or reminding the reader about. In most all situations, parentheses won't appear inside dialogue. When they do appear in dialogue, parentheses usually denote something that has not been spoken aloud by the speaker; rather, they are used to express the speaker's thoughts or provide some extra information. Using parentheses inside dialogue is very rare, and shouldn't be added just for the heck of it.
How Are They Used?
Parentheses quite literally isolate a single phrase, sentence or thought from the rest of the words around it -- and you don't have to use any special punctuation before or after using a parenthesis. The parenthesis by itself is a punctuation, so you don't want to double up unless you're ending the sentence directly after the parentheses are closed. With the exception of separate thoughts and sentences within the parentheses, no punctuation should appear inside the parentheses. If you're ending a sentence immediately after the parenthetical text ends, always put the period outside the parenthesis. Some examples might make it easier to understand:
Johnny and Anne appeared to be in love (though they'd broken up about a dozen times in the past).
Notice the period placement in the above sentence. If the period were to appear before the final parenthesis, it would be woefully incorrect. However, punctuation may sometimes be necessary even inside parentheses. Example:
Johnny and Anne were fighting again, and everyone was holding their breath to see what would happen next (and save their wind for the rush toward the nearest exits, an exodus that would surely begin once the argument escalated).
Without the comma that appears in the middle of the parenthetical text, that phrase isn't going to look pleasant or read correctly. However, you'll notice that the period of the sentence still appears outside the parentheses. Remember, only punctuate inside them if it's relevant to the specific phrase that's being isolated -- normal punctuation rules still apply to the sentence as a whole.
When used sparingly, parentheses can be a powerful tool for making text stand out and getting points across to the reader. Like every other kind of special punctuation, however, overuse of parentheses will have a negative effect on the entire book.