Much to my disbelief (no, I'm kidding, it happens all the time), I found out that my handbook was wrong to tell me we must use 静けさ, because 静かさ doesn't exist. Wiki(tionary) says "For degree, ...
To say new, the first way that I learned is to use the i-adjective 新しい. But I noticed that the na-adjective 新たな is used a lot in written texts. Is there any difference in when either is used?