I’ve decided that I’d like to write a fan letter to my favorite author. According to her official web site, in 1991 she was created Baroness James of Holland Park. I’ve never considered writing to anyone with this kind of title before, so I don’t know what salutation to use.
Is there some weird etiquette rule regarding this title, like “only use it in person; in writing just address her as ‘Mrs.’ or ‘Ms.’?” Or, aren’t there some titles that are only used formally (i.e., even in person I wouldn’t call her ‘baroness’)?
Could it possibly be as simple as “Dear Baroness James?” 
Baroness
CORRESPONDENCE: On the envelope, (i) if a Baroness in her own right “The Rt Hon the Baroness_____” or socially “The Baroness_____”, or “The Rt Hon the Lady” or “The Lady_____”; (ii) if the wife of a Baron “The Rt Hon the Lady_____”, or socially “The Lady_____”. Commencement, “Madam” or socially “Dear Lady_____”.
PERSONAL ADDRESS: “Madam”.
See also Baron’s Widow.
N.B. If a Baroness in her own right marries a commoner and has issue, the children have the same rank and are addressed as if their father were a Baron.
From Debretts
“Dear Lady James!” Excellent! Thank you!
(Side note: I love the phrase “has issue” for “has children.” :D)
Oh yeah: Yes, I’ve decided to take a certain amount of liberty and use the social form of address. “Madam” without any part of her name just sounds too formal – like I’m applying for a job or something. I trust that the author won’t judge any breaches of ettiquette too harshly.
Well, I did briefly consider “Phyllis, bubbula!”
I prefer a standard, “Hey, bitch,” or in this case, “Hey, Bitch.”
Terms of endearment are cute and make you seem warm and approachable. Trust me.