I hang out with my friend Ms. Ruth Smith every friday for a drawing session. This one week I drew her yet again (I draw her every week). Lately I've been playing around with a more detailed style of drawing using lines to communicate shadows and form. I was mentally referencing the drawings of Albrecht Durer and Peter Paul Rubens as I did this drawing. I say mentally referencing because I had no physical book with me, only the memory of they're works that happened to be on my mind at the time.
This is why it is a good idea to collect a library of books and constantly study the works of your favorite artists of past and present. The more you study the more you will remember and be able to call on that information for reference when you need it, even if no hard copy sources are available. I don't usually drawing in this style though I have experimented with it many times in the past. I can't put enough emphasis on routinely trying out new and different styles of drawing, it helps to get out of monotony and keep it a fun and playful experience.
Tip: If you find yourself having a bad drawing day, try a different style or media.