This is a great question without a solid answer. Your history book is not necessarily outdated, but since there is an on-going debate about this issue, one can find a proponent that will argue any position taken is wrong. The core of the question here is whether the spread of Indo-European languages represented a diffusion of people or languages. Internationally, many people use English today as a lingua franca without any trace of genetic material tracing back to the Angles, Saxons, and Jutes who settled Britain in the fifth century. Languages (and religions) diffuse without being part of the cultural baggage of immigrants. At the same time, people do move, and there are historical examples of mass migrations of people, so we can't automatically rule out the spread of at least some speakers of Indo-European languages.

The "widely disregarded academic view" is the idea that enormous numbers of Indo-Europeans spread, conquered, oppressed (and to a certain extent eliminated) indigenous populations. This has been discredited because of a lack of clear evidence and because it is simply hard to imagine, but the "discrediting" was linked with a refutation of the sort of racist worldview that contributed to the rise of the Nazis - so there was added inspiration to drive a nail into that coffin.

If we can step back, however, and attempt to look at this whole thing objectively, we can conclude a few things. The first is that there is far too little evidence to determine the numbers of people who moved. There is archaeological evidence of something going on (the so-called beaker people, provide one example - but material culture may also be a matter of the diffusion of fashionable objects rather than people, and there is no way to link a language to an object), and there is the linguistic evidence - clearly the languages did diffuse. There is also a bit of cosmological evidence since there are similar pantheons with supernatural beings with similar names found in far removed places. Does this add up to people moving? Not necessarily, and yet people do move, so we cannot discount the idea that at least some people moved to some places. Did this occur in large numbers? This is the question at the heart of the controversy, and to a certain extent, the question rests with the definition of the word "large." Some people probably moved around, but the idea of enormous numbers arriving to oppress local populations is harder to imagine. The numerical answer, then, is some between "some people" and "enormous numbers."

This is also a great question for /r/linguistics/ which can probably even better answer this in an adequate way.