Hot off the Press! These are not your eldritch manuals or ancient folios of doom. These are random books, a GM can use whenever a PC reaches for a shelf, which usually happens when said GM is least expecting it. These can be found in most libraries, many private homes, and anywhere else one could expect to find a book. More than a few have some built-in plot hooklings as well, while others, like many books of our age, are plain drivel.
Darklings are a kind of Halflings living in the sewer of medium to large cities.
Even though they smell badly and are quite dirty, Darklings are friendly and clever.
They try to limit the knowledge about their existence to only a small number of trustworthy persons, including player characters.
Darklings trade in information. Money, weapons and magical items have no value for them. This is why they hide beneath the sewer grates, listening to all that is said in the city above.