How do I improve my house?
I received a book that told me I need to talk to someone to improve my house in Whiterun.
Who is this person, and where can I find them?
I was so avid for an answer, misreading the question as "how do I improve my **mouse**"?
The same person you bought the house from also sells the furnishing for it. For the house in Whiterun (Breezehome) this person is Proventus Avenicci, who you usually find near the Jarl.
If he's not near the Jarl, he'll be on the Grand Porch. Took me forever to find him .:)
Note: I've found that the option to buy upgrades isn't available when he is in his quarters.
i've found that the option to buy upgrades wasn't available at all until he was in front of the throne.