Down to Ques Querax is the first part in the Cyclopean Deeps campaign and is loosely connected to the Rappan Athuk campaign. You don't need Rappan Athuk to play the campaign.
The book provides the overall hex map of the dungeon, encounter tables, basic information on dungeon access, movement, monsters, and underground caravans. It also details the first areas of the dungeon that leads the characters from the entry point to the underground fortress of Ques Querax (which is detailed in the next part of the series).
From the publisher's website
The first adventure in the Cyclopean Deeps mini-campaign, Down to Ques Querax introduces the perils of the Under Realms with encounter tables, region map, and many new monsters. Designed as the "link" to Rappan Athuk, this module may also be used to connect any deep dungeon to the Under Realm.