I am very happy about this book, I first read it in 1979 when I was 19, and I found it really marvellous. I agree with the other reviewers, but I must add a note of caution - the edition I have contains some errors. They are as follows;
p155, p157 - the factors w(11) and w(10) are incorrectly placed on the butterfly diagram 10.3 and 10.4 respectively,
p166, p168, p169, equations 10-26 and most equations following to the end of the chapter - the factors R(N-n) and I(N-n) should be R(N-1-n) and I(N-1-n), respectively.
I hope I'm right about this, but the convention is that the indices are from 0 to N-1, and therefore if n=0, then N-n is N - which not an allowed index.
Apart from these sort of errors (I havn't been through the whole book with a fine toothcomb), its really very good, actually extraordinarily clear.
One of its main benefits is that it doesn't veer away from the FFT to the very complicated developments such as fractional transforms and other developments which might confuse the sort of audience it's aimed at (which is definitely the graduates).
But if you want to look deeply into FFTs for a real application you will need a lot more. I must mention,for instance, that the implementation of an FFT needs fairly careful error propagation and rounding analysis, and this isn't covered at all in the book. Neither are prime factor FFTs. In fact the chapter "FFT algorithms for arbirary factors" is only a method of factoring into powers of two, and certainly not the prime factor decomposition which was developed later by Winograd, Chuo, and others.
It must also be said that while the DCT is practically a kissing cousin of the FFT, this naturally isn't covered in this text... but neither are the finite field implementations that are now taking many peoples imaginations to faster and faster FFTS.
Also, there are jolly useful things to know about, such as the FFT when you only need a subset of the output data points. There are pruning algorithms which greatly simplify the computations.
But it's very good as a starter, I wouldn't do without my copy!