The World in Our Dreams by Kevin Geoffroy

Her name is Beatrice, and she set out from a sheltered life to experience the world. But the world she found made he anxious and afraid, so she retreated back into her mind. While daydreaming, she creates wonderful structures, vast libraries, trusting companions, and impossible landscapes. Her days become a waiting period for to go back into her dreams, until something appears in her dreams that she did not create. It is sentient, it is aware of where it is, and it hints at something far more dangerous than she ever imagined.