Betty Edwards produced a book many years ago called "Drawing on the Right Side of the Brain". It is a series of drawing techniques and lessons that is very popular and helpful to novice artists. Since then she has revised her drawing program considerably. It can be very helpful for those who wish to learn to draw realistically.
Another classic book on drawing is "The Natural Way to Draw" by Kimon Nicolaides. It is a bit 'old school' now but has been used to teach drawing in fine art schools for many decades.
But there are many approaches to drawing. Not sure what you mean by "draw by numbers"... If you draw too systematically your drawings will look mechanical, not natural and they will lack unique style and feeling.
Learning to draw takes a lot of practice. If you are bored with your own drawings try looking at different books on drawing to see the variety of drawing techniques. You might try using different materials, too. IOW, if you mostly use a pencil, try a pen or a brush. But most importantly try to not judge your drawings... the point is to draw and draw and draw and keep drawing and keep learning new things as you draw.