When you open a book and read the beginning of the first page and it is so fantastic that you just have to keep on reading you have experienced a successful hook. Hooks are essential in writing, but are more than just a marketing tool. In fact, your whole being as a writer revolves around hooks, because a good writer doesn't put a great hook at the beginning of a story and then just hope the reader never loses interest. A hook must extend beyond the first line, into the entire paragraph, and then more hooks need to be present in each paragraph, and each chapter, inspiring the reader to not only want to continue reading, but making it difficult for the reader to set down the book.
Hooks should not be used as simply a marketing gimmick, or overused in a dramatic way. A hook must also, never, stand on its own. A manuscript must not teeter on the fulcrum of a one-liner. The hook must be a part of the text, leading the reader into the next portion of the conflict, further hooking the reader to read to the next hook, and so on. Hooks ought to populate the openings and closings of line breaks. Treat each paragraph as if it is the opening of your book, and as if whether or not the reader continues to read your story depends on the effectiveness of the hook and the intensity of each paragraph.
Have other writers read your hooks, and the rest of the work as well. Is the hook effective? Does the reader want to read each line, and each paragraph, clawing their way through the pages to see what happens next? If so, you've succeeded.