Note: I rarely buy books for myself. There are only two "always buy" authors I can think of for myself---Barbara Kinsolver and Sue Monk Kidd.
In my library there is a different story. There are several authors whose books I will automatically order as soon as they are published:
1. Rick Riordan
Even though his intended audience is middle graders, high school readers still love his books and will wait in line for a turn at them.
2. John Green
After The Fault In Our Stars was published Green's popularity went through the roof. Even though I had copies of his other books I had to go back and get extra copies of them and will automatically order whatever he publishes next.
3. Sarah Dessen
Her popularity has waned a bit the last few years but I still automatically ordered her most recent book which was published this summer.
4. Chris Crutcher
I am a Crutcher fan so make a point of always ordering his latest book.
5. Marissa Meyer
The wildly popular Cinder series causes me to automatically order the next book of the series when it is available.
6. A. S. King
Ms. King isn't the most popular author in the library but she is spectacular writer so I always end up ordering her books.
7. John Flanagan
The Ranger's Appentice series was so popular, students would warn me months ahead of its publication date to be ready. Now that that series is finished, popularity in the companion series has waned. I think it is really geared toward younger readers.
8. Carl Deuker
A Washington author, he is a sure bet with male readers.
9. Andrew Smith
One of the zaniest authors alive, Smith's books appeal to certain type of sophisticated reader. I just finished his latest, The Alex Crow. It is weird indeed.
10. Maggie Stiefvater
A personal favorite. I love everything I've read of hers and promote her books heavily.