Okay, first of all let me get this out of the way: Absolutely read the Discworld Books.
You don't have to read them in order, but I'd recommend them as there's subtle references planted in later books to events in prior books, and I would definitely recommend reading a series within Discwork in order, if possible.
There's a general 'rule' about reading them 'in order' if you're into that sort of thing - start with the first book of a protagonist you like, and read forward from there all the books they're in. There's: Rincewind: An inept wizzard (note spelling) who sometimes travels with an idealist named Twoflower, almost always travels with an intelligent and malignant luggage, tends to be paired with the Wizards of Ankh-Morpork, and always, always runs away from his troubles, constantly, forever, and somehow it works out anyway. Start with "The Color of Magic" and "The Light Fantastic", which are the first two books in Discworld, though are both a bit more 'goofy fantasy' than the latter books.
The Watchmen: Specifically Samuel Vimes, but this follows the city guard of the principal Discworld town of Ankh-Morpork, as they evolve to try and keep up with the demand for city protection in a world with constant influxes of magic and literary references. A lot of then play out like fantasy detective novels, which is great. Start with "Guards, Guards!", which contains dragons.
Death: Terry Pratchett's Death appears in almost every book he wrote, and is, bar none, the best grim reaper ever. He also has a daughter named Susan who takes over for him sometime, but he alone is a great enough character that I'd highly recommend reading these books. Start with "Mort", where he takes on an apprentice.
The Witches: Especially Esme Weatherwax and Nanny Ogg. Think of them as essentially the answer to all of the stereotypes of witches that have ever existed, turned on their head, and magnified tenfold. Granny Weatherwax is a no-nonsense woman who takes no prisoners and gets things done. Nanny Ogg has 30 children (or more) and acts exactly like you'd expect an old lady who has had that many children to act. There's also some other witches that come and go, and this branches into the YA "Tiffany Aching" books. Start with Equal Rites, but keep in mind that it's a bit less about the witches than the rest of their books, and is practically non-canon with the events that take place later, so consider starting with "Wyrd Sisters" instead.
The Wizards: They don't really star in their own series, but they're *very* prominent in other series. Over time, the wizards of Ankh-Morpork have become their own set of unique characters. Worth looking into if you like wizards, no particular book to recommend as a 'first' though.
Moist Von Lipwig: Comes in MUCH later in Pratchett's novels, a former con-man who makes an 'honest living' at odd jobs in the city. Not sure how I'd characterize this one, but it's pretty good. Start with "Going Postal".
The History Monks: A group of monks charged with protecting the very nature of time and space, the greatest of which, is Lu-Tze, an old man who mostly dispenses Ice Cream Koans and uses the trope of 'never fight an old man with a broom' from martial arts movies to prevent people from taking him on. But also uses time travel. Start with "Thief of Time", though this also comes much later in the series.
There's also some side-books that borrow characters from the rest of the series, but don't necessarily follow any of these groups: "Pyramids", "Moving Pictures", "Small Gods" are just a few of them. And again, while you don't have to read them in order, you do gain a bit of 'I recognize these characters' from doing so.
Oh, and he also wrote a neat trilogy that is NOT based in a fantasy setting, and is more sci-fi ish, about a young boy whose active imagination (or maybe not?) transforms the world and people around him. I'd call them "Johnny" books, because the latter two are called "Johnny and the Dead" and "Johnny and the Bomb", but the first one is called "Only You Can Save Mankind", and is one of the best reads I've ever had.
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Okay that was a lot longer than I expected, but moving on:
I recommend "The Hobbit" over "Lord of the Rings", because it's the correct size (one novel, instead of three tomes) and is much more lighthearted book specifically intended for younger audiences (Tolkien wrote it for his kids).
I also recommend "The Hitchhiker's Guide to the Galaxy" trilogy, a series of five books that is on the 'very absurd' end of sci-fi. Like, even moreso than El Goonish Shive. Yes. It's actually more absurd than El Goonish Shive.
And for a more serious fantasy setting, I might recommend the "Wizard of Earthsea" books...admittedly I only read the first one, but it's good.
I'd be remiss not to recommend some non-fantasy, non-scifi literary classics. "Catcher in the Rye" is a very unique (but very adult) take on story-telling, entirely told from first-person perspective, entirely with a fully-biased first-person view. "One Flew Over the Cuckoo's Nest" is equally adult, and is a very engaging and, frankly, horrifying look into what the mental health system was like circa the middle of the 20th century (it's thankfully improved since then). "Lord of the Flies" is even more horrifying than those two why am I suggesting all of these sad books?! Um, "To Kill a Mockingbird" is all about growing up in the racially-divided south but is actually a pretty nice book about a pretty nice man. There's lots of other literary classics I'd recommend, but they're escaping my mind at the moment.
TLDR: I think Dan would really like the Discworld books, I think you ALL would really like them, and also some other suggestions.
