The following paragraph is from the novel the Moon and Sixpence.
Dr. Weitbrecht-Rotholz belongs to that school of historians which believes that human nature is not only about as bad as it can be, but a great deal worse; and certainly the reader is safer of entertainment in their hands than in those of the writers who take a malicious pleasure in representing the great figures of romance as patterns of the domestic virtues.
I understand the italic part as the reader who reads the book of that kind of historians would feel more comfortable. But I don't get the of. There is no safe of in my English dictionaries and if it is binded to entertainment, a noun seems to be positioned in front of of.
What is that of?