I've actually read all of Rick Riordan's mythic world books in the Percy Jackson and Heroes of Olympus Series, so that's 10 books right there, and their side books, and frankly, that universe never got old for me. I've also got the first 2 books in the Trials of Apollo spinoff, also connected to Percy Jackson. Sadly, his Magnus Chase books were so meh for me, mostly because I didn't like the protagonist, Magnus, AT ALL. I thought he was really whiny and self-pitying and didn't really have anything going for him. Which is a shame, since I love Norse mythology and I was hoping he'd crack into it eventually.
I also read half of the Princess Diaries (got bored by book 5), some of the House of Night (got bored by book 2), read all of Harry Potter but prefer fanfic now due to all the plotholes/deus ex machina in the original series, read the first book of Beautiful Creatures, but was SO bored, tried reading Pretty Little Liars, didn't really like it at all.
I'm so persnickety about what I read and a book series, especially a long book series, has to be engaging.
I'm amazed they didn't include Tamora Pierce's Tortall Universe books there, since they included Rick's Percy Jackson/Magnus Chase mythos. She has 17 main books in the Tortall series (Song of the Lioness Quartet, Immortals Quartet, Trickster Duology, Protector of the Small Quartet, Beka Cooper Trilogy) and that's not including her short stories and whatnot. I love and own ALL of these books, some in multiple copies because the old ones wear out, and it's the one series I've read with 17 (almost 18, Numair's book is coming out next year!) books that has never gotten old or stale for me. So yeah, a bit miffed about her not being included XD
But still a great and fascinating list though! As a writer, I'm sure I'd struggle keeping things fresh for my readers for 19 books. Dang.