In the Light, Beneath the Shadow is a puzzle book for children, and is more importantly one of the five Quest Items that you need to complete the Collection of Dragon and Snakes World Quest. To obtain this book, you must have completed The Subterranean Tales of Drake and Serpent, which will allow you to switch Enkanomiya from day to night, and vice versa.
Start by heading to the Dainichi Mikoshi during Evernight and jump down until you find the Ruin Grader beside the explosive barrels.
Attack the explosive barrels to blow up the wall behind them, revealing the entrance to a hidden room. The Ruin Grader will also wake up, so be prepared to fight it.
Upon entering the hidden room, you'll encounter two triangular mechanisms, a gate, and a stone wall. Your goal is to move these structures to reveal the code hidden behind the stone wall. While it's still Evernight, Attack the triangular mechanism A (the one glowing blue) to unlock the gate.
After, head outside. You'll find a day-night switching mechanism on a pillar in a crater straight ahead. Activate it to switch Enkanomiya to Whitenight. Then head back inside the hidden room and Attack triangular mechanism B to move the stone wall. Once that's done, go outside and switch Enkanomiya to Evernight again.
Back inside the hidden room, you'll find an Exquisite Chest in front of a blank wall. Climb the wall and interact with it to reveal symbols (only accessible during Evernight). The dots beside the symbols indicate the order in which they should be activated in The Serpent's Heart. It's a similar puzzle to the one in The Heart of Ouroboros.
Head to the clearing in the temple in The Serpent's Heart. Exit the transparent walls from the inside out in the same order as the image above. You must do each step from the inside out and not from the outside in; otherwise, the puzzle won't complete.
After successfully passing through all five walls, a cutscene will play and a Common Chest containing the Quest Item In the Light, Beneath the Shadow will appear.
Reference our guide on How to complete The Heart of Ouroboros in Enkanomiya Genshin Impact if you're having trouble solving this Hidden Exploration Objective.