The Librarian of Unseen University is one of the most popular characters in the Discworld series to the extent where pin badges bearing the legend Librarians rule Ook are now available. The character appears or is mentioned in most books in the series.
The Librarian first appeared in the second novel of the series, The Light Fantastic, in which he was transformed into an orangutan as a side effect of a powerful magic spell. Discovering that being an orangutan has certain advantages for a librarian, he refused to be transformed back into a human, and has remained an orangutan ever since. Most of the grimoires are highly dangerous to people, and sometimes the books even consume them, but the Librarian is no person, and he no longer requires a ladder to reach those tricky high-up shelves. He retained his position as librarian despite his condition because "he's the only one who knows where all the books are" and the fact that he could screw a man's head off with his feet. The other wizards have gradually become used to the situation, to the extent that "if someone ever reported that there was an orangutan in the Library, the wizards would probably go and ask the Librarian if he'd seen it." (Night Watch).
Death's Domain is a dimension in the Discworld novels, shaped by human expectation and Death's own attempts to have a life beyond his allotted task. It gave rise to a Discworld Mapp of thesame name, drawn by Paul Kidby, with additional material by Pratchett and Stephen Briggs.
The first thing visitors notice is that the Domain is black. Everything in it is black (or different colours and shades of black), except the things that are bone-white. The exceptions are purple mountains in the distance and, since Reaper Man, golden cornfields between the mountains and the Domain proper. Very few living people have entered the Domain, but among the notable exceptions are Albert, Ysabell, Mort, Rincewind, Susan, Twoflower (and, arguably, the Luggage).
Things that are black include all the plants in the garden, the house, the peacocks (although they have white skull-shaped "eyes" on their tails), the cats (Death likes cats) and the bees (Death also likes bees, possibly because a hive mind has no fear of him). Things that are bone-white (and indeed skeletal) include the trout in the pond, some of the birds and the garden gnomes. Death cannot create, he can only copy things. The items in the garden are things he has seen elsewhere, including the fish pond, and presumably the fish inside it.
Although nearly everything in the Domain is black, it is not all the same colour. On the Discworld, and congruent dimensions, splitting darkness with an eight-sided prism produces different colours of black.