Linguist John McWhorter presents a very strong case for his belief that Celtic, Welsh, and Viking influences changed English grammar. In the first half of the book, he points out that Anglo-Saxon English had no instances of meaningless "do" or progressive present tense (still unlike any other Germanic language) until it came into contact with Celtic and Welsh, which do contain those grammatical oddities.
The book is clearly not written for an academic audience, but I found myself a little irritated by how overly simplified it sometimes was. The fact that McWhorter rarely names the linguists whose positions he is refuting seemed a little disingenuous. He makes vague mentions of "one renowned scholar" or "the pre-eminent linguist in this field", etc. But where I hoped to find easy references for their work, there were few immediately available. The "Notes on Sources" section at the end contained everything, but while reading the chapters there was no way to easily cross-reference. I like my footnotes, damn it!
In the middle, I got bored by an attempt to discredit the Whorf hypothesis. And the book ends with a theory that Proto-IndoEuropean (Proto-German's ancestor and thus English's as well) changed its grammar because of proximity to ancient Semitic languages. I appreciated the round-robin ending: that syntactical changes are part of the history and nature of English and languages in general.
Despite the brief annoyances, I couldn't put the book down. McWhorter presents such a compelling case and writes in a semi-snarky tone that makes linguistics accessible for armchair linguists. Plus, the idea that the Celts most significantly influenced how the English language developed? SOLD!