The Game Master: A Guide to the Art and Theory of Roleplaying

From the publisher: The Game Master is an eBook about roleplaying games. It is for anybody who runs or is thinking about running RPGs to discover how to consistently produce games that work and avoid train-wreck campaigns, and to gain a deeper appreciation of the special nature of RPGs as both a literary form and a type of entertainment. It is provided on a “Pay What You Want” basis, and is free to distribute. Contributions to the author are heartily encouraged if you find the book useful, but are not required.