This is undoubtedly from a book of prayers, possibly from a form of Breviary.
The first half reads:
Deus in adiutorium meum intende
Domine ad adiuvandum me festina
The Gloria Patri is a very common prayer, so only the first words of each sentence are written. The full prayer is:
Gloria Patri et Filio et Spiritui Sancto
Sicut erat in principio et nunc et semper et in saecula saeculorum. Amen
Both Deus in adiutorium and Gloria Patri are common building blocks of longer prayers, as in the Roman breviary.
What is written in red are rubrics: liturgical instructions to the readers rather than part of the prayers. The first rubric reads ad tertiam which is one of the hours in which the praying of the breviary is usually divided.
The second half is part of a hymn, Memento salutis Auctor (English translation in the linked page):
MEMENTO, salutis Auctor,
quod nostri quondam corporis,
ex illibata Virgine
nascendo, formam sumpseris.
Maria, mater gratiae,
Update: pulling the thread in Wikipedia I got to another article using the image giving a deeper description.
The book is indeed a Book of the Hours (breviary) known for its inventory name Milan, Biblioteca Trivulziana, Codex 470. The picture on the left is represents the annunciation to the shepherds (right after Jesus was born, cf. the Gospel of Luke, ch. 2.)