Every book on how to write always mentions a hook. You need to hook the reader with the first lines. The first Sentence? The first Paragraph? The first Page?
The reader needs to form a question in his/her mind? A question they want an answer to, so that they turn the page to find it.
Opening a book with action may not be such a good idea. The reader needs to feel for the protaganist. Why woud they care who wins that exciting battle with swords if they don't really know who they should be
Others have mentioned that a reader opens the book to read the first page to see if they want to spend their hard earned money on purchasing that book. An opening hook is important. I, myself, read the flap and open to the middle and start reading. I want to know if the middle is any good. Am I the only one that does this? I can't be. There needs to be "hooks" throughout the story.
The Fire in Fiction by Donald Maas, tells us there should be tension on every page. I agree. Keep those inner thoughts and dialogue revved with tension. Make the reader care about the hero, question the antagonist motives, worry about the characters.
There are 250 words on a page. When I open a book I am looking at 500 words, in those 500 words there better be something that intrigues me. Do I want to read on to find out or go back and read what happened before?
Remember to keep the tension, the questions, and the worry in the reader's mind. When they stop reading to cook, clean or answer the phone, they think of the story, they need to return to it to find out what is happening.