Let me start by explaining what I mean by "milestone books". Books that were crucial in one's development as a reader, helped define tastes, etc, that are still remembered fondly despite flaws or changed tastes. I hope I explained it well. Maybe there's a more professional term for such books, I'd like to know if that's the case.
Here's how it was for me.
My first milestone book (or rather series) was Harry Potter. I was 11 when I started reading it. It was pretty much at the peak of Harry Potter mania - just after The Philosopher's Stone movie. Before HP I didn't really like reading, I didn't find it enjoyable. But I devoured the first 4 books in the series within a couple of weeks and I couldn't wait for the premiere of the 5th one. So I was hooked and realised how much fun reading can be.
At the age of 15, I read The Lord of the Rings. As a kid, I had a vivid imagination and I started my reading journey with Harry Potter so it shouldn't be surprising that fantasy became my favorite genre. LotR was my first "adult" fantasy book, completely different than Harry Potter much more complex and with proper fantasy worldbuilding.
Next was Hyperion by Dan Simmons. I got this novel for Christmas in 2007. I consider it important, because, well, it was my first SF novel and before reading it I was adamant that SF isn't for me because it's too scientific. I know it's silly, but I was a teenager. I remember how my heart dropped when I realised it SF novel, but I had to try at least reading it because it'd be rude to do otherwise. After finishing Hyperion I no longer claimed that SF isn't for me. I still preferred fantasy but I didn’t shy away from SF.
Soon after I read The Witcher series by Andrzej Sapkowski. It was my first polish fantasy series (I'm from Poland) and it prompted me to see what the native fantasy market has to offer. It was also my first fantasy series with morally grey characters and more serious topics that were served in an easily digestible way.
After a year or so I started reading books authored by Nobel prize laureates. I don't really remember, why I decided to do so. Because it was quite a significant leap from fantasy. I guess that I did it at least partially thanks to "The Doll" by Bolesław Prus - a polish classic from the realism era. And I read it because, well, I was mandatory in Polish classes. I didn’t like it at first, I found the book so boring that I was falling asleep. But then I read a chapter about Napoleonic wars and I was hooked. Why? I don't really know. It was just a retrospect, and the main plot of the novel was, as most novels of the realism era, tackling complex social issues of the 19th century. Nevertheless, from that moment I enjoyed the book thoroughly. I also found it much better than the books of Henryk Sienkiewicz - a contemporary of Prus and the first polish author who was awarded the Nobel Prize in literature. In my opinion, he was George R. R. Martin of 19th century Poland and didn’t deserve the prize at all. Not when compared to Prus. I sulk to this day over that injustice.
Like I mentioned I started reading some books by other Nobel Prize laureates (Orhan Pamuk and Doris Lessing) but I wasn't really impressed by them. But soon before I turned 19 I read The Street of Crocodiles by Bruno Schulz and soon after Ferdydurke by Witold Gombrowicz. Those two books were revolutionary for me, because before I didn't realise how enjoyable can be to read something surrealistic without any coherent story, but full of extraordinary writing. And that was it - my final milestone books. Ever since I have preferred to read books that are experimental in one way or another, with exceptional writing and stories that aren't necessarily important. Of course, I read much more books between the ages 12 and 19 but they weren't as impactful as those listed above and ever since I read tons of great books, but, funny enough, I don't remember some of them at all. But my milestone books I remember well even though I read them only once or twice more than 10 years ago. That's also the reason why I think they're "milestones".
Some of my reasoning may seem silly, but this is exactly the way how I was thinking as a teenager and I'd like to hear your funny stories from the same age :). #entertainment source