Significant work gets professionally published by academic publishing houses with review boards and such. I'm not going to say that independently published work is always of little or no value, but I will say that it is not anything you would want to cite in your own work, or rely upon for understanding in a field in which it is so easily ignorable due to the abundance of properly-vetted, academically-rigorous material (it's not like Historical Linguistics is a new field, after all). The fact that I recognize the name of the person whose quote is being used to sell the book (German linguist Bernd Heine, who is as famous as a specialist in Grammaticalization can be) but not the author himself is a bit concerning. I would steer clear of this work, at least until you have a more solid foundation in the subject. To put it in terms that are probably more relatable to the majority of this website, since you don't all have linguistics degrees: if given the chance to read from, say, the KJV or a Bible of similar provenance, or from a hot-off-the-presses, independently-published 'new' translation by someone whose name you do not know and whose credentials you cannot independently verify, which do you think you would depend upon and make most use of? Maybe eventually you'd look at the latter (for comparative purposes, or maybe just out of curiosity), but you'd almost always choose the former first, right?