Congratulations! You've completed Braid! Has the princess been saved? What was Tim's purpose in this journey? The ending leaves a lot up for interpretation, but that is the quality of Braid. The game is about the journey, not the ending. On this level you will find a bunch of books giving more insight to what has been going on, but in the end the story is for you, the player, to make your own conclusions about. You might notice that some books are red while others are green, and how most green ones contain no text. There's actually hidden passages in every red book that reveal a bit more to the story, and in true Braid fashion, they require you to solve a puzzle to find them.
The first part is not difficult at all. Climb up to the red book next to the key. Read the text for a bit of back story. Now, for the hidden passages to appear Tim is required to hide himself from view and face away from the red book while it's open. When done correctly, a woman's voice will sing a tune. Reading a green book will cause the red one to close, so you need to get in hiding without crossing the green books' path. To the right of the red book there's a large boulder. A perfect hiding spot for Tim. Move him behind it and the song should play and the new passage will appear instead of the old one. Read it, grab the key and continue on through the gate.
Keep going right here and you'll fall down to the second red book; open it. There's a nice sheet to hide behind at the top of the ladder, but a green book is blocking it off. Begin to rewind time instead, and come back up from where you fell. Hide behind the ottoman and look left and new text should appear. Read it, go climb the ladder and continue on.
There isn't much to interact with this next room. Go to the red book and take your time reading it. Then go past the green book (it doesn't matter that the red one closes) and continue to the next room. Here you'll find a platform with green glow that will transfer over to Tim. Go stand on the platform and carefully rewind time. When you hear a book page flapping, that means the green book you passed in the previous room has unflipped, and the red book is open again. Stop rewinding and go back to the previous room and hide behind the statue of a boy. Read the new passage and go back to the room with the green-glowing platform.
See the lever over at the right hand side? Go pull it. It will make the green-glowing platform rise up to the ceiling. Let it rise and then pull the lever again to bring it back down. Now go stand yourself at the red book; read it if you haven't already. Then rewind time and the platform will rise with Tim still on it. When reasonably high up jump down the left side and hide behind the large boulder on the bridge. Read the final hidden passage and continue on.
When you reach the end of the Epilogue level you'll find a castle built from blocks of all the levels you've traversed; built from Tim's journey you might say. There are green books here which actually contain text, so make sure to read them. When you choose to exit through the door, you'll be taken back to the outside of Tim's house. If you haven't sneaked a peek at the Stars section already, now is probably a good time to do so. There might be more to Braid than you thought there was.