The path to the tomb leads through the cave on the edge of the map. Before you enter, prepare some stronger weaponry, because there is a bear inside. After you kill it, punch a hole in the wall and go down the path. In the chamber, collect a Relic and run to the very bottom.
You will get into a location where you can dive into water. Turn towards the rocks and collect an Explorer Satchel. Close to this place (at the back) you find a Relic, and among rocks on the right, there is a Survival Cache. Follow the path, underwater and between rocks - at the other side you resurface by the camp (next to the campfire, there is another Survival Cache). You can also dive here into the chamber to the right of the camp. Here, you find an Archivist Map and a Survival Cache.
Swim over to the other side and start climbing down. In the spot, where green gas is escaping, shoot a burning arrow to blow up the gate.
In the next chamber, you find two Documents (on the ground level on the right and a balcony on the left) and a puzzle. First, shoot a rope arrow towards the two levers above the ground level. This raises the valve and releases a green gas into the chamber. Jump down quickly, where you find a basin opposite the gate, which remains partially filled, temporarily. Run towards it and release the hanging cage. Do this before water floods this area.
Approach the mechanism and pull the cage to the surface. Run up, behind the cage and shoot the rope towards the mechanism- - this way you move the cage towards the gate. Finally, release gas again and shoot at the two levers tied with the rope. Finally, shoot a flame arrow towards the escaping gas - the explosion destroys the gate, thanks to which you can read the book.
Reward: Ability "Greek Fire" (increases damage from flame arrows and from molotovs).