From what I can see, biographies of Horace do not abound – information to the contrary is greatly appreciated – but I found one at a library today when I searched for a print edition of the works of Horace:
Jacques Perret: Horace
with a foreword by Jotham Johnson
translated by Bertha Humez
New York University Press, 1964
AbeBooks.com has a number of inexpensive copies.
A glance through the book indicates that it is very readable. Perret, a professor of Latin at the Sorbonne, has written about Vergil too. JSTOR has several reviews indicating that the book is written for a general audience, i.e., us; three out of four reviews are favorable, with some reservations which probably would pass by the general reader.