How we cite our quotes: Citations follow this format: (Act.Scene.Line). Line numbers correspond to the Norton edition.
Quote #1
ORSINO
Stand you a while aloof.—Cesario,
Thou know'st no less but all. I have unclasp'd
To thee the book even of my secret soul.
Therefore, good youth, address thy gait unto her. (1.4.13-16)
Here, we learn that Duke Orsino has shared some private thoughts about his passion for Olivia with his page, "Cesario." It's not surprising that Orsino should align his feelings of desire ("the book of his secret soul") with a text because the Duke sees himself as a kind of poet. We see him regurgitating clichés from famous books of love throughout the play and in Act 1, Scene 5, we learn that Orsino sends "Cesario" to Olivia to recite a love poem/letter before the Countess.
Quote #2
VIOLA
Alas, I took great pains to study it, and 'tis
poetical.
OLIVIA
It is the more like to be feigned. I pray you,
keep it in. (1.5.192-195)
Olivia quickly shoots down "Cesario's" attempts to recite Orsino's love musings, but why? Viola is certainly impressed by the Duke's passion so why doesn't Olivia think Orsino's great? The obvious answer is that Olivia's just not attracted to the Duke. But, here, we also see that Olivia thinks that carefully written or studied poetry is fake and insincere.
Quote #3
VIOLA
Most sweet lady—
OLIVIA
A comfortable doctrine, and much may be said
of it. Where lies your text?
VIOLA
In Orsino's bosom.
OLIVIA
In his bosom? In what chapter of his bosom?
VIOLA
To answer by the method, in the first of his heart.
OLIVIA
O, I have read it; it is heresy. Have you no more
to say? (1.5.221-228)
This passage picks up and develops the idea that one's passionate feelings are like a "book." (Compare this to 1.4.2 above.) When "Cesario" tries again to recite Orsino's message to Olivia, the Countess calls him out and says she's "read it" all before. This not only reveals that Olivia really has read Orsino's love musings before, but it also suggests that Orsino's lines are unoriginal and can be found in just about every book of love poetry there is.