Well it depends of what you're looking for. Background? Rules? Adventures?
Ruins of Hyboria is indeed a good choice, being the best book of the line for me along with Across TheThunder River.
Of the regional sourcebooks (Thunder River, Aquilonia, Stygia, Argos & Zingara and Shem) Across the Thunder River is the best (unless you don't plan to send your players on the Pictish border), although one might find some interest in Stygia.
The Hyboria Series (Fiercest, Finest and Fallen) are generally appreciated by D20 players and can prove useful if you plan to delve deeper into the rules. The books focus heavily on multiclassing options. Not really my cup of tea.
Of Skelos, Pirates and Free Companies, Secret of Skelos (the new edition) would probably be my pick, although the books tend to be more useful for high levels NPCs than for players. Pirate Isles would be next. I found little interest in Free Companies (eventhough I was running a mercenary campaign at the time!)
Black Kingdoms, Heretics of Tarantia, Lurking Terror of Nahab, Tower of the Elephant, Reavers of the Vilayet are adventures. Tales of the Black Kingdoms is my favourite (although it needs some rewriting), followed by Heretics of Tarantia. Tower of The Elephant is well playable, especially if your players don't know the story before hand. Reavers is just OK and Lurking Terror is terrible. There are also two other adventures available only in PDF from the Mongoose site. I found them only average.
I forgot Tito's Trading Post, a lousy cut & paste compilation of stuff and weapons from other books. Avoid it.
Don't forget to check the Signs & Portents online magazine from Mongoose. most issues contain Conan stuff, and it's free!