This is an out of print book from 1965. It is often for sale as a used book and often goes for several hundred dollars. I spend time at libraries reading it.
I have read Feynman’s QED which I think is one of the best popularizations of physics in many years because it actually tells you many important things about QM that you don’t need to unlearn when you get to the next book. It does this with remarkably few equations. Feynman omits certain details in QED and omits different details in QMPI. QMPI omits relativistic QM but includes spin and such. QED omits spin but does things relativistically with remarkably little hassle. QMPI is heavily into math and the intent of the book is to reveal the tricks, mostly found by Feynman, to compute answers with quantum electro dynamics. I suppose that the tricks are not dependent on relativity. Or at least that the tricks you learn on non-relativistic calculations need not be unlearned.
I have read several recent popular books on the dire straits of modern particle physics. Each tries to give a taste of the nature of modern particle theory. There are several questions that I had that were not answered but are addressed in the introduction to QMPI. The most significant so far concerns the nature of amplitudes. Amplitudes are these mathematical descriptions of states that are not observable, but which must be calculated in order to make the successful predictions of QM. Feynman says:
A more accurate equation valid for electrons of velocity arbitrarily close to the velocity of light is the Dirac equation. In this case the probability amplitude is a kind of hypercomplex number.I take this to mean that the hypercomplex numbers, such as Clifford numbers, supplant the complex numbers of non-relativistic QM. So far it is possible to know what sort of value the expressions take on in Feynman’s book. I do not generally find this to be the case in QM books.
(2011) There is now an affordable Dover edition of this book, emended by Styer!