You went to a talk given by an author last night. You are describing the speaker, who you didn't enjoy very much. You say:
She wasn't that engaging of a speaker.
This is an expression that you use when you want to say that something (like a "person") doesn't have very much of a quality or characteristic (like "niceness"):
not that nice of a person
The meaning of this is similar to:
not a very nice person
But "not that ___ of a ___" is more casual.
An "engaging" speaker is one who interests you and makes you feel involved. He or she keeps your attention.
You can also call a book "engaging". It would be strange to call a movie "engaging" unless it was an intellectual kind of movie, like a documentary.
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