Glimmering Tablet: The Gatekeeper's First-Kill Drop
The 
Glimmering Tablet is Eternal Starlight's in-game guide book, and it drops from The Gatekeeper the first time you defeat it. There is no crafting recipe and no other source, so if you are looking for the tablet, you are really looking for that boss.
How you get it
The Gatekeeper's loot is written so the tablet only appears on your first victory over it. Beat it again and the tablet is not in the drops, because the game has already given you one. The same first-kill rule applies to the 
Orb of Prophecy, which drops alongside it.
That pairing is deliberate. The orb is the key that opens the portal frame and the focus you cast spells through; the tablet is the manual that explains what to do with the orb and everything after it. Killing the Gatekeeper hands you both halves at once.
If you lose the tablet, it is not gone forever - it is an ordinary item, so it can be recovered from your death drop or handed over by another player. Just do not expect a second one from a repeat kill.
What is inside it
The tablet is not a static book. Chapters unlock as you progress, and the game tells you when it happens with a Chapter Unlocked notice, or Chapter Updated when an existing entry gains new material. Opening the tablet after a milestone is usually worth doing, because the new chapter is normally about whatever you just did.
The index covers the mod's tools, mechanics, creatures and plants, including entries for:
Seeking Eye and the locator items that make it point at a specific structure- Orb of Prophecy, portal opening, crests and spellcasting
Crescent Pendant and the accessory system, which combines items into equipment with accessory slots
Energy Transmitter and 
Accumulator, the mod's two redstone devices
Drying Rack, 
Nocturnal Millet, 
Parasol Grass and 
Starlit Lily Pad for the survival side
Malarite Sword, 
Dagger of Hunger, 
Aetherstrike Rocket and 
Seeds Launcher for combat
Tooth of Hunger, Tangled Skull, 
Frozen Tube, 
Coldsnap and 
Crystalborn Catalyst
Pungency Fruit and 
Silver Pungency Fruit, the Dark Swamp food line
Abyssal Magma Block and 
Chiseled Golem Steel Block
Several chapters carry combat notes rather than recipes. The Starlight Golem entry, for instance, spells out that its armour only becomes vulnerable while it is absorbing energy, and that putting out the energy blocks it is drawing from is what opens the window. Reading the relevant chapter before a boss fight is genuinely faster than learning the pattern by dying to it.
Reading it in practice
Open the tablet from your inventory and the index gives you a chapter grid; each entry is a page of illustrated notes with the recipes and item displays built in. Because chapters unlock with progression rather than all at once, the book doubles as a checklist - a locked entry is a thing you have not found yet.
What to do next
The natural first step after the Gatekeeper is the Orb of Prophecy entry, since the orb is what opens the way onward. From there, the Seeking Eye chapter is the one that saves the most time, because a seeking eye held with the right locator item points you at a specific structure instead of leaving you to search the dimension by hand.
Covers the mod's in-game guide topics: