There definitely are earlier references than our book. An asymptotic formula for
$\sum_{p \leq x} p^a$
is in T. Salát and S. Znám, On the sums of the prime powers, Acta Fac. Rer. Nat. Univ Univ. Com. Math. 21 (1968), pp. 21-24. (Cited by Spearman & Williams--I actually have not seen this paper.) It probably goes back further than that. The natural place to look would be Landau's "Primzahlen"--I forget the exact title--but I was unable to find that sum in there.
Eric.