A book's success is linked heavily to the reviews people leave on places like Amazon. So here's an explanation of what those stars for the reviews actually mean.
5 star = you liked / loved the book and you will read the next one
4 star = you liked the book, but you probably won't read the next one
3 star = you completely disliked the book and definitely won't read the next one
2 star = you hated the book
1 star = you really hated the book and wish you hadn't bought it
What suprises most people is that Amazon counts 3-star reviews as negative reviews.
The best way to help out an author you like is to give them a written review along with a positive star rating, especially if you bought (rather than borrowed) the book as these reviews are given more weight by Amazon. Even just a quick "I enjoyed it" with 4-5 stars can make a huge difference to how many people Amazon will suggest the book to.
To leave a review:
It's as simple as that, and you'll have helped that author considerably, (especially if it's a 4-5 star review).