I first watched the movie in 2001 while on vacation at the age of seven. I didn't understand much of the movie as it was in Japanese with subtitles that I couldn't understand either. The images alone, however, were enough to make a lasting impression on me (I couldn't sleep for months without the light on). Last year, after a slowly rekindled interest in horror movies, I decided to watch Ringu again with my friends (this time with proper subtitles) and found myself very intrigued in the story and also by Nanako Matsushima--the queen bee of Asian drama, but that's beside the point. Needless to say, once I found out that the film was based on a trilogy by writer Koji Suzuki, I had to get my hands on the books.
Ring, Spiral, and Loop, respectively:
Fortunately for me, the books were already translated into English by Glynne Walley. I got a hold of Ring, Spiral, and Loop (the first three of the trilogy) around August of last year, but unfortunately that coincided with the start of senior year and college apps for me so I never got around to reading them. Recently, however, I found myself with a lot of idle time and finished Ring while vacationing and Spiral in a single sitting during a plane ride. I finished Loop about an hour ago. It's rare enough to find a book these days that I can read in a single sitting--finding an entire trilogy of books that are so difficult to put down is beyond amazing. I'm punching myself for not ordering Birthday at the same time when I bought Ring, Spiral, and Loop.
If you find yourself remotely intrigued by the premise of these stories--beginning with the famous deadly videotape, then prepare yourself for a journey of nothing less than shitting bricks. You don't have to purchase paperback versions--I believe all of the books are available in ebook form. Honestly, in hindsight, the movie serves as a very poor introduction to the series because its style is so different from that of the trilogy and it grossly bastardizes the most crucial of details and plot devices. The way the story unfolds itself cannot be done justice with summary so I will avoid any of that here.
TL DR; These are amazing books that everyone should read. Mindfucks everywhere.