The 5 secret texts are clearly labeled in the book pages, with dark orange text highlighting the respective passages. You then need to translate them from the game language into french and then (for me) into english.

After all that, you get the clues. Don't read further if you want to decode and solve on your own and just wanted a starting spot!

If you want more details on all of the page translations, or the alphabet, check out this great guide:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3295106212

You'll notice small 3x3 grids throughout the pages, each with a small symbol nearby + an x in one of the squares. This creates an 8-digit code. The code can be input onto a keypad found on a robot in level 1-9. Note that the robot is lying on its side, so the keypad is rotated 90 degrees.

The input order is: Left Middle, Left Upper, Left Middle, Right Lower, Right Lower, Right Middle, Middle Upper, Left Lower.

There is a big square rock machine/creature on 2-1. Click on its eyes about 5 times.

You need to pull a purple vegetable out of the ground. It's found on 3-3 in the lower right corner. There is a small bit of dark purple visible, with 3 dark gray leaves sticking out of its top. Click on it about 50 times to pull it out.

You have to turn the lights off in the house found in 4-6. Click on the window about 10 times, until the mushroom create comes out and yells at you.

The clue translates to "on a rocket, on a single colored letter cube, to then believe in your star".

You need to input a melody on the device found on 5-8, with the symbols on it. Each one is a note.

On the page from 2-4, there is a rocket with an A symbol on it. There is also an F symbol on a cube on the page for 2-6. The last 2 notes are R A, which I brute forced. So 1st key, 3rd key, 2nd key, 1st key!