Galaxy Quest (Vintage Tie-In Paperback) by Ellen Weiss

For years, the intrepid crew of the NSEA Protector valiantly entertained millions of television viewers with their voyages through space. Now the crew is on another adventure-bravely touring the sci-fi convention circuit and making guest appearance at two-bit computer stores. But their dull existence gets a quantum rocket boost when a group of aliens-believing the television episodes to be true "historical documents"-enlist the crew's help to save them from an evil alien empire. Now these washed-up TV actors find themselves deep in space, playing their roles as if their lives-and the lives of an entire alien race-depend on it!