Not much for the 13th Doctor yet in terms of novels (and no audios yet), but I'm sure the list will grow:
BBC New Series Adventures (Novels) The Good Doctor The Molten Heart Combat Magicks
BBC Children’s Books (Novels) The Secret in Vault 13 The Maze of Doom [due out Nov 2019]
OPTIONAL: COMICS, SHORT STORIES, MISC. Short stories, e.g.: - The Missy Chronicles (The Liar, the Glitch and the Warzone) - Twelve Angels Weeping (The Rhino of Twenty-Three Strand Street) - Thirteen Doctors, Thirteen Stories (Time Lapse) Comic strips: DWM and Titan Comics are both producing 13th Doctor strips, so I'm sure we'll be able to utilise some concepts. In fact, the latest issue from Titan Comics (The Thirteenth Doctor #9) features a character many people have been hoping we'll be able to include in the sourcebooks:
he latest issue of the Titan series gave us the first appearance of the Corsair.
The above information is a major spoiler as this is not revealed until the final panel of the issue. It was already included in spoiler tags in the OP, so I have added them to your post. I imagine that it'll be fairly general knowledge after the next issue is released, but at the moment it's a spoiler.