All his life, my dad has fed an SF book-habit (also classical music on vinyl) comparable to Imelda Marcos' penchant for shoes. So he bought most of the DW novels as they were published, and (having graduated from Douglas Adams) I read them all off his shelf at least once during my early/mid-teens. (I also read the first 8 or so of Robert Asprin's 'Myth/M.Y.T.H.' books, but I always liked Pterry's stuff better).
From my early twenties onwards (once I was working!), regular and diligent scouring of the local secondhand bookshops has allowed me to (re)acquire most of the main-sequence DW novels for myself (in paperback). More recently, those from Going Postal onwards were delivered as Christmas/birthday presents (so in hardback, although I still prefer paperbacks). I'm missing 'only' Moving Pictures, The Fifth Elephant, Thief of Time*, and The Truth, and all the Tiffany Aching books except A Hat Full Of Sky (I could have sworn I also had a second TA novel, but none of the synopses on Wikipedia sound familiar).
*I did actually acquire secondhand copies of ToT and TFE -- among others -- during the 18 months I was working in Dahab,. Unfortunately, I did not own a Luggage! So to lighten my extremely excess baggage, just before I left I passed on all the paperbacks that I'd accumulated to a friend. I still occasionally kick myself about that...
I know I could almost certianly locate/order the missing titles over the internet, but some of the fun of this little project is the physical search itself: so resorting to Amazon (quite apart from all the problematic financial/ethical aspects) just feels like cheating. Also, while browsing an actual bookshelf, I often find other books/authors that pique my interest (as opposed to having things shoved at me by a 'smart' algorithm).
I also have GoOd oMeNs, all 5 of The Long... books, the Bromeliad, Johnny and the Dead, Nation, A Blink of the Screen, and Dragons at Crumbling Castle.
I've never read FaustEric or The Last Hero -- mostly because (to the best of my recollection) I've never encountered a copy of either of them. And though I remember not liking Strata or The Dark Side of the Sun very much as an early teenager, I wouldn't be averse to buying/ re-reading them now, with the benefit of 30+ years' (mis)education behind me. (I might well even have read Strata before Ringworld, so some of the jokes/ parodies/ satire in there would have whooshed straight over 12 y.o. me's head). Dodger is also on my to-read list, if I can find a copy.