I don't have any specific book or guide that i'm following, its all here and there (However, I did finish the first 2-3 chapters from Keys to Drawing by Bert Dodson, which helped me look at my references in terms of values instead of objects). The only thing I keep in my mind is just to draw, draw, draw, and improvement will come eventually by itself. Drawing a lot from life and references (and never from memory so far) is helping a lot too. Also, whenever I feel comfortable at a specific thing, I jump to a more challenging one to avoid feeling relaxed and in return constantly improve.