how does Mr. Shelby describe Tom? Why does he think Tom is a valuable slave?

From the text:

Tom is an uncommon fellow; he is certainly worth that sum anywhere,—steady, honest, capable, manages my whole farm like a clock."

"You mean honest, as niggers go," said Haley, helping himself to a glass of brandy.

"No; I mean, really, Tom is a good, steady, sensible, pious fellow. He got religion at a camp-meeting, four years ago; and I believe he really did get it. I've trusted him, since then, with everything I have,—money, house, horses,—and let him come and go round the country; and I always found him true and square in everything." ch 1