Yao Guai Tunnels

The Yao Guai Tunnels are located due west of the Charnel House, and northwest of Smith Casey's Garage. The entrance is a chain-link fence door in a rocky outcrop. As expected, the caves are inhabited by a number of Yao Guai.

Since all of the enemies here are animals, the Animal Friend perk will cause them to ignore you.

Loot
There is a Pre-War Book on the picnic table just as you enter the Tunnels.

In the southern section of the Yao Guai Tunnels there is a Grognak the Barbarian book along with a dead Raider on a high ledge on the south wall (accessible as you exit from the "Yao Guai Den"). Also the Pip-Boy map shows more southern section area that does not seem to be accessible.

There is also a bottle of Nuka-Cola Quantum sitting on a picnic table in the northern part of the Tunnel map.

Bobblehead Location
To find the Sneak bobblehead, turn north at the first major intersection and follow the tunnels to the door to the Yao Guai Den. Once inside the Den, walk straight ahead to the central pillar, and then bear (heh bear) left. Immediately look for the natural stone ramp that leads along the eastern edge of the cavern, overlooking a crescent-shaped pool of water. At the base of the ramp is a Metal Box with the bobblehead standing on top of it.