Toibin takes a haughty writer of old and tells a simple story - allowing the reader to get to know Henry James to the point of forgetting you're reading about a famous author until another character refers to him by name. This fictionalized account of Henry James' life focuses on James' personality. James is just as confused as the rest of us. Throughout the novel he questions his sexuality, battles the guilt he feels over a friend's suicide, serves as caretaker to his dying sister, and struggles to find his place in the world. His biggest emotional battle in the book is his fear of literary failure. Toibin skillfully humanizes James by speculating that he wasn't as confident a person as his prose suggests, making this pseudo-biography quite an interesting and entertaining read.
You don't have to be familiar with James' body of work to read The Master, but it might make you want to check him out afterward. I suggest you start with The Bostonians.