The Destiny 2 Lighthouse secret chest is one of the more interesting aspects of the new Mercury patrol space.
Until now Region chests in the main game weren't too difficult to find, but Curse of Osiris has one locked behind a seemingly impenetrable forcefield, right next to where you find new vendor Brother Vance.
Akin the bell puzzle in Rise of Iron, gaining access requires you to activate a series of switches in order to access it.
How to start the Lighthouse secret chest puzzle with the Compelling Book
Head to where Brother Vance is within the social part of the Lighthouse (it's through the teleporter to your right when you travel to Mercury) and as soon as you enter, drop down to the left to find the forcefield and the chest behind it.
You'll only need to remember where this is at the end once it's open, but it's good to have this frame of reference.
To get started with the puzzle, go to Brother Vance. With the door you entered behind you, to the left of Vance is desk on the outside edge of this central area.
There is a book here with a glowing symbol that can be interacted with, displaying the prompt 'Examine Compelling Book'. Interact with this to start finding the remaining books to complete the puzzle.
Sign of Osiris puzzle solution to unlock the Lighthouse secret chest
Whether you want to discover the rest by yourself, or follow via the specific directions below, remember that you have a short amount of time to find each following book, otherwise the puzzle will reset. You have unlimited attempts, however, so don't sweat it if you mess up a couple of times.
All of the following use the doorway you enter through as the point of reference, with each subsequent book alternating between left and right.
Marked Book location
From where you enter, this is on the far right, on the outer edge. Find the two burning torches, and the glowing book will be inside the bookcase in-between them.
Aged Book location
This is to the left as soon you enter, just past the first column. Look at the bookcase and you'll see it just tucked in at the wall.
Curious Book location
From the right of where you enter is a long bank of desks curving round. Toward the far end, the book is sandwiched in-between other books. It's a little awkward to get to, so muscle your way in until the prompt appears to activate it.
Significant Book location
The final book is on the left side, just past the large, stone symbol adorned with lines and red dots.
It is high up on the book shelf, so you'll have to leap up against it, landing on the thin ledge below. Crouch to activate it.
Final step to unlocking the Lighthouse secret chest
Once you have done the Significant Book, you're not quite done yet. The final step is to decrypt a firewall - in order words, activate another prompt.
This is found on the right side, almost in-between the second and fourth books, out in the open. It's a large glowing conflux that is very hard to miss, so just run up to it and interact when prompted.
Doing so will open the forcefield to the chest. Again, it's located to the left of where you begin, by dropping down and turning right, but if you're stuck an on-screen marker should assist you.
Inside we found a Legendary Engram and a handful of planetary Tokens. This is probably not worth the effort, but it's one of those cases where the puzzle itself is the reward - and at least gives you something to do in Mercury other than repeat the same Public Event over and over again.
It's worth adding that although you cannot open the chest more than once, you can repeat this puzzle as many times as you like, if you fancy repeating it.