I’ve read many instruction books, Roger Love is one of the best. Here is a tip from his Web site:
The best way to develop a unique style is to spend time learning to imitate great singers. Pick a different singer every week and try and “become” that voice. Play their music over and over, sing along, trying to sound exactly like that artist. Then, record yourself singing along, and play it back listening to how close you have come. If you don’t sound the same, go back to listening, and then singing along and recording yourself later. Follow these simple steps until you’ve become a great imitation of the original artist. Then pick another singer, and start all over again. Sometime between now and “who knows when”, you will learn to mix and match those imitated sounds, and create a unique vocal style that is old and new, familiar and fresh, and it will ultimately help you achieve the next levels of success.