For me, one of the earliest books that totally sucked me in was The Wonderful Wizard of Oz, this had a lot to do with the original illustrations I feel, but the detail in those books amazed me, like the city and people made of china, if you've never read the books and only seen the film, please check out the books!
The other was The Lord of the Rings, which is the standard (for me) in creating a complete fictional universe, if you consider the Silmarillion is a large appendix, part of what made the world of LOTR so deep was the references, songs, poems and tales to Tolkien's First, Second and early parts of the Third Age. And as remarkable as the society was in the present in those books, you had the feeling (or at least I did) of melancholy for the cities and civilizations that were ruined at that point.
YET, the background is not the story, and never overwhelms the story (in Tolkien), there are times I find pretty cool ideas in novels, movies, comics, and then the characters don't do much to engage me, so while background and worldbuilding is important to me, you also have to have characters that engage and excite you.
The other was The Lord of the Rings, which is the standard (for me) in creating a complete fictional universe, if you consider the Silmarillion is a large appendix, part of what made the world of LOTR so deep was the references, songs, poems and tales to Tolkien's First, Second and early parts of the Third Age. And as remarkable as the society was in the present in those books, you had the feeling (or at least I did) of melancholy for the cities and civilizations that were ruined at that point.
YET, the background is not the story, and never overwhelms the story (in Tolkien), there are times I find pretty cool ideas in novels, movies, comics, and then the characters don't do much to engage me, so while background and worldbuilding is important to me, you also have to have characters that engage and excite you.