1. J.K. Rowling abandoned another book that she was writing in order to start working on the Harry Potter series.
2. The series wouldn't exist without Alice Newton, the then-8-year-old daughter of the chair at Bloomsbury Publishing, who read the first book before it was picked up for publishing and called it "so much better than anything else".
3. There are only 500 first-edition copies of Harry Potter and the Philosopher's Stone in existence: around 350 hardback copies, and 150 proof copies.
4. In contrast, Harry Potter and the Deathly Hallows had the highest initial print run in history, with 12 million copies released at midnight on July 21, 2007.
5. In 2014, a full set of first-edition Harry Potter books sold at auction for £11,250.
6. The Harry Potter series is the biggest-selling book series of all time. As of 2011, it had sold more than 450 million copies.
7. According to Rowling, in the Potter-verse, there are about 3,000 wizards living in Britain.
8. In 2006, Rowling said that "The Mirror of Erised" is her favourite chapter in the first book, and one of her favourites from the entire series.
9. She has since said that her favourite chapter in the series is "The Forest Again" from Deathly Hallows. Both chapters involve Harry interacting with the memory of his parents.
10. The character of Rubeus Hagrid was based on a Hell's Angel who J.K. Rowling met in the West Country. According to Robbie Coltrane, "He was just huge and terrifying. And then he would sit down and talk about his garden and how his petunias had been very bad that year."
11. In Philosopher's Stone, Hagrid says he bought Fluffy from "a Greek chappie" he met in the Leaky Cauldron. Three-headed dogs are prominent in Greek mythology, the most famous being Cerberus, who guards the Underworld.
12. Despite this connection to Greek mythology, in the film version Hagrid says he bought Fluffy from "an Irish fella" instead.
13. In the Harry Potter universe, a gold Galleon is worth around £4.93 ($7.35), a silver Sickle is worth around 29p ($0.46), and a bronze Knut is worth about 1p ($0.02).
14. That means that, in the first book, Harry paid £34.51 – or $51.45 – for his wand at Ollivander's.
15. Since its original publication in 1997, Harry Potter and the Philosopher's Stone has been translated into around 70 different languages, including Latin and Ancient Greek.
16. Non–English-speaking fans became so impatient when waiting for translations that they began translating the books themselves.
17. In 2007, a 16-year-old was arrested in France for publishing a pirated translation of Harry Potter and the Deathly Hallows online.
18. In 2002, an unauthorised fifth book was published in China titled Harry Potter and Bao Zoulong. The title was translated into English as Harry Potter and Leopard-Walk-Up-To-Dragon.
19. The book consisted of the text of J.R.R. Tolkein's The Hobbit with names changed to Harry Potter characters. The first line was "Harry doesn't know how long it will take to wash the sticky cream cake off his face."
20. Other unauthorised sequels have included Harry Potter and the Crystal Vase and Harry Potter and the Golden Turtle in China, and the Indian Harry Potter in Calcutta.
21. Even the American versions of the books were translated into American English. "Fringe" became "bangs", "ice lolly" became "ice pop", and "letterbox" became "mail slot".
22. In the French edition, Voldemort's middle name is Elvis. The change was necessary so that his full name – Tom Elvis Jedusor – would become an anagram of "je suis Voldemort".
23. In the Spanish edition, Neville's pet, Trevor, has been translated as a turtle instead of a toad.
24. In 2009, the family of late author Adrian Jacobs filed a £500 million lawsuit against J.K. Rowling, claiming that she plagiarised a significant amount of Goblet of Fire from Jacobs' The Adventures of Willy the Wizard. The lawsuit was dismissed in 2011.
25. J.K. Rowling has said that Voldemort's boggart would take the form of his own corpse, because "he thinks that [death] is a shameful human weakness".
26. And if he stood in front of the Mirror of Erised, he'd see "himself, all-powerful and eternal".
27. If Hermione looked in the Mirror of Erised, she would see the trio "alive and unscathed and Voldemort finished".
28. Hermione's Patronus was an otter, which also happens to be J.K. Rowling's favourite animal.
29. Drawing the cover art for the British edition of Harry Potter and the Philosopher's Stone was illustrator Thomas Taylor's first ever professional commission.
30. The illustration took two days to complete, and Taylor never got a hardback copy of the book.
31. Rowling decided that the Hogwarts Express would depart from King's Cross station because her parents met on a train to Scotland from King's Cross.
32. J.K. Rowling had to learn a lot about alchemy and folklore while writing the Harry Potter books. The Hand of Glory, for example, is inspired by folklore dating back hundreds of years.
33. Rowling explained that Dumbledore lived to be over 100 years old because "wizards have a much longer life expectancy than Muggles".
34. Dumbledore is an old English word meaning "bumblebee". Rowling said she named the character that because she "always imagined him as sort of humming to himself a lot".
35. Rowling has said that, if she needed to tell her readers something, she would have either Hermione or Dumbledore say it, because Dumbledore "knows pretty much everything anyway" and it's plausible that Hermione has read it somewhere.
36. The word "Muggle", meaning a non-magical person, was added to the Oxford English Dictionary in 2003.
37. The conductor and driver of the Knight Bus – Stan Shunpike and Ernie Prang – are named after Rowling's grandfathers.
38. Early drafts of Harry Potter and the Goblet of Fire included a character called Mopsy Fleabert, an author and dog lover who took Sirius into her home thinking he was a stray dog.
39. In Greek mythology, Cassandra was a woman who was gifted the power of prophecy by the god Apollo. After she refused to marry him, he cursed her so that her prophecies would never be believed. Professor Trelawney's grandmother was named Cassandra.
40. Rowling has likened the all-important prophecy regarding Harry and Voldemort in her series to the prophecy made by the three witches in Shakespeare's Macbeth.
41. She has also said that it "amuses" her to allude to historic events within the Harry Potter series, and that it is "no coincidence" that Grindelwald was defeated in 1945.
42. J.K. Rowling actually wrote the epilogue to Harry Potter and the Deathly Hallows "in something like 1990".
43. According to Rowling, the death of Hedwig in the final book symbolises the loss of Harry's innocence.
44. She also seriously considered killing off Ron "out of sheer spite" about halfway through the series, but ultimately decided that she couldn't do it to the character.
45. When writing the epilogue to Harry Potter and the Deathly Hallows, Rowling considered having Dudley Dursley show up at Platform 9¾ with a magical child, but decided that "any latent wizarding genes would never survive contact with Uncle Vernon's DNA".
46. The famous ending to the series – "all was well" – was almost entirely different. J.K. Rowling said both in 1999 and 2005 that the last line of the series was "scar".
47. Rowling has said that she won't write any prequels to the series, because readers "don't need them".
48. She has also said "categorically" that she will not write another fantasy novel after the Harry Potter series.
49. However, she has said she has considered writing an encyclopaedia of the Wizarding World in order to "rid [herself] of every last lurking detail. So there's still hope.