To find the tomb entrance, start in the Ruins Encampment Base Camp and go North, along the river. Walk under the rocks and in the distance, you spot a path to the cave. Inside, jump over to the next ledge and squeeze through the rift - this way, you will find the camp.
Jump over to the other side again. On the right, next to the brook, there is an Explorer Satchel. Keep down and be prepared to take a jump and to jab the hatchet into the stone wall, when Lara starts to slide down. A bit ahead, walk over the fallen tree trunk (on the left, you find a Survival Cache). Follow the narrow corridor and jump over to the other side - Lara falls down here, straight into the main part of the tomb.
Swim over to the other side (in the corner on the left, there is a Survival Cache). Then climb onto the platform on the right, over the tracks (as seen while you are facing the water reservoir). At the top, pick up the Relic and the Document.
Give the turning wheel, to the left of the cart, a spin by 180 degrees. Thanks to this, you can push down the cart. In the recess on the right, you can jump into a small chamber, where you find an Archivist Map. Finally, use the zipline next to the cart that you have thrown down and take the ride to the next one.
Behind the second cart pick up a Relic. Climb to the top (the passage on the left, where there is another Survival Cache at the wall) and use the turning wheel to lower the platform to the bottom. Once there, give another turning wheel a spin to orient the tracks towards the cart.
Push the cart onto the platform and switch the tracks again, to make them face towards the elevator's turning wheel. Only then, take the ride up, on the elevator, with the cart, where you need to push the cart down. This will unlock the gate at the very bottom, across which you can enter the chamber with the book (in there, you also find a Document).
Reward: Ability "Geologist" (you can obtain Chromite Ore).