Wait, I see people talking about this sometimes and it's actually a slightly trickier question than it seems at first. There are seven main novels, that's the straightforward part. 'Harry Potter and the Philosopher's Stone', 'Chamber of Secrets', 'Prisoner of Azkaban', 'Goblet of Fire', 'Order of the Phoenix', 'Half-Blood Prince', and 'Deathly Hallows'. That's the core series you read in order.
But then the whole list gets fuzzier if you start counting ancillary works. There's 'The Tales of Beedle the Bard', 'Quidditch Through the Ages', and 'Fantastic Beasts and Where to Find Them' – which are real published books but exist in-universe. Then there are the screenplay books for the 'Fantastic Beasts' films, and even 'Harry Potter and the Cursed Child', which is a script. Some collectors argue those belong on a 'complete works' list, while purists stick to the seven novels. I personally only count the seven for a proper series read-through; the others are bonus material you can dip into after. The main journey is absolutely those seven books, in that exact order, no skipping around.
Ultimately, for someone asking for 'the series in order,' the answer is seven. Giving them a list of fifteen things including screenplays and in-universe textbooks just muddies the water when they're probably just trying to figure out what to buy or borrow first.