Amazing.
Ash has written a trilogy of math books on subjects related to elliptic curves, culminating in this excursion into number theory. Introducing Riemann zeta function, Bernoulli series, and the divisor function (by means of various standard generating function techniques). Ash and Gross start in Chapter 8 and 11 with their mission proper, the upper half plane, q, linear fractional transfromations, SL2(Z) and notions of modular form. Providing just enough proofs, they are a splendid introduction to the wonders of a field for a reasonably diligent mathematically willing reader to stop and admire the many blossoms of the number theory flower garden, as Freeman Dyson put it somewhere.
I found it convenient to start my reading at the modular form chapter and go backward and forward to sample the other delights.
Easily a serious math writer for the semi popular or semi serious math educated public all in a expository league by himself.