very short answer;
Jarlaxle is a character in the Dungeons & Dragons-based "Forgotten Realms". He first appeared in the novel, Exile, by R. A. Salvatore.
Bregan D'aerthe is an elite mercenary band (guild) who's leader is Jarlaxle.
Some books to read by R. A. Salvatore;
The Icewind Dale Trilogy
The Dark Elf Trilogy <------ Jarlaxle first appears in second book of this trilogy (if I remember correctly, long time ago)
Legacy of the Drow
Paths of Darkness
The Cleric Quintet
The Hunter's Blades Trilogy
The Sellswords
Transitions
War of the Spider Queen