Basically "Gateway" books/authors are the books and authors that got you into reading (or back into reading) or reading a genre you never though you'd read.
Top Seven "Gateway" Books/Authors In My Reading Journey:
I remember that I kind of stopped reading in fifth grade for a while. Then my sis forced me to read this book and I was like, OH YEAH, I LOVE READING and started getting back into reading, lol.
This series really got me back into reading after I was done with Harry Potter.
After I finished that chronicle, I basically devoured every book that he wrote that was in my school's library. I think he introduced me to the high fantasy genre.
I think Twilight was my first YA book. I mean, that book was basically what spawned the YA paranormal genre.
These two books were what really got me interested in book blogs. I was looking for some YA books to read online when I ran into a few book blogs, and their reviews for these two books had me really interested. After being blown away by them, I started getting really into book blogs, because I wanted to continue reading awesome books.
This series was what got me into Shojo manga. I never looked back ever since.