Online encyclopedias such as the Internet Encyclopedia of Philosophy, the Stanford Encyclopedia of Philosophy, and Wikipedia offer overviews of philosophical problems and theories regarding knowledge. There are also many introductory texts, including:
Charles Landesman, An Introduction to Epistemology (Wiley-Blackwell, 1996)
Robert Martin, Epistemology: A Beginner’s Guide (Oneworld, 2010)
Jennifer Nagel, Knowledge: A Very Short Introduction (Oxford, 2014)
The story of Molly and Doug and the burned-out light bulb is known as a “Gettier Problem,” named for Edmund Gettier and an article he published in 1963. The Gettier Problem led to a great number of papers providing new JTB+ accounts or raising new problems to the new accounts. To get a sense for this extensive literature, you can simply search for “Gettier Problem” on the internet.
A very good general account of Gettier problems, and how they just won’t go away, is Linda Zagzebski, “The Inescapability of Gettier Problems,” The Philosophical Quarterly, vol. 44, no. 174 (Jan., 1994), pp. 65-73.