It's a tricky situation -- Harry Potter and the Half-Blood Prince is a strong entry in the series about the boy wizard, but won't work as a stand-alone film. No background is provided, so audience members must have seen the previous movies or read the J.K. Rowling books or they'll be lost. Fortunately, the movie doesn't assume you've read the sixth book -- there are no gaps in the story that require the novel to fill.
In case you haven't read the book, Harry Potter (Daniel Radcliffe) returns to Hogwarts in Half-Blood Prince for his sixth year of study, while Death Eaters terrorize both the magical community and the ordinary Muggle world. Dumbledore (Michael Gambon) trusts Harry with increasingly important tasks in the fight against Voldemort ... and Harry is also learning a lot from an old Potions textbook that contains clever notes from someone self-styled the "Half-Blood Prince." Harry's best friend Ron (Rupert Grint) is taking lessons in Quidditch, self-confidence, and romance -- and why is Hermione (Emma Watson) displeased? Draco lurks in hallways and towers looking pale and conflicted.