English Translation
The library is usually quiet.
Should we use estar in this case rather than ser?
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Interesting question!
My take on this is that the word suele tells us that this is more or less the permanent state of the library, so using ser is appropriate.
Of course, it would be nice to have a native opinion.
English Translation
The library is usually quiet.
It’s because the difference between ser and estar isn’t whether the adjective is “permanent”. (Our teachers say lots of things that aren’t true.) “Estar” comes from the Latin verb that gives us “state” and “status” in English (stare, “to stand”). In Spanish, “estar” describes a thing’s state of being.
On the other hand, “ser” is from the Latin “esse”, which meant “to be,” and therefore describes the thing’s very being, something about its identity, or something innate to it. There’s room for disagreement over whether an adjective describes a state of being or something more fundamental, but not here.
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I suppose I should have used another wording than “permanent state”, maybe “inherent quality”. 
I wouldn’t say your word choice here affects my answer. Since “soler” just denotes tendencies, it doesn’t do anything to imply that the property is somehow inherent in the noun. Using it with “estar” in this case means the library tends to be in a certain state. (What would be interesting would be a sentence using “ser” with “soler”. I have yet to see one anywhere.)