when u beated/kill the big plant u will see a sparkling. Its the helmet key use it on the other doors in the mnsion that is lock. So go back to the mansion.Then when ur inside thers is a door on the left next to the room were u have to figure it out "Cradle to Grave" U need the helmet key to open that door, use it. When ur inside u will c a lamp on top of the table.Turn it on then, u will see the doom book 1 on the desk and a magnum bullet on the bookshelf.

Read where it said doom book 1

In Rebirth, you have to go back to where you killed Plant 42. The Yawn has returned and
you have to kill it in a 1st person knife battle (Meep). Kill it and it will drop the Doom
Book 1.

You have to go back to plant 42 room where you will find a snake that you will beat in
knife battle, then you'll find the doom book 1.