Just some remarks from a very specific angle.
Being Dutch but living in the U.S., I usually tell my American friends that for Anglophones Dutch is the most related language (actually Frisian is even more related but that mini-language is Holland's second language, and I don't want to make things too complicated). The funny thing is that even well-educated Americans normally are not aware of this fact. Some even told me that Celtic, being the second language of the U.K., was probably more related (this book makes clear, by the way, that Celtic is related to the Germanic languages but only remotedly, as another member of the Indo-European family), or French (because of the many loan words).
Well, this book confirms what I tell my Anglophone friends. The funny thing is that I am quite convinced that the Old and even Middle English texts, which Barber quotes, are probably more comprehensible to Dutch readers than to Anglophones. Dutch remained a fully West-Germanic language but English, though at its core still very much a West-Germanic language, was heavily influenced by North Germanic (Scandinavian) and especially French, due to the invasions of Vikings and Normands. It's curious how the Old English texts use words that still are current in Dutch but (more or less) lost in modern English: "onfeng" is "ontvangen" in Dutch but became "receive" (i.e. a French loan word) in English. "Niman" is "nemen" in Dutch but "take" (Scandinavian) in modern English. "Witan" is "weten" in Dutch but became "to know" in modern English. I found endless other examples in this book ("Ic dorste" = Ik dorst (in Dutch) = I dared (in English). In fact, I guess that an old question I had can now be answered. I was always curious to know what the Anglosaxon missionaries Willibrord and Bonifatius in the 7th and 8th century spoke when they brought the Gospel to the Lowlands. Well, it seems pretty sure that they just used there own language that was still intelligible to the Low Landers (something like Dutch and Afrikaander these days: it takes a little bit of effort only to understand it). Some of the Old English texts read like a modern Dutch dialect.
I have, though, the feeling that Barber does not know much Dutch (nor Frisian). We hear something about Dutch loan words that ended up in English in later centuries. I wonder, however, whether some of the modern English words, that according to Barber have Scandinavian roots, are not just West-Germanic. Why is "give" Scandinavian, if the Dutch cognate is "geven". Same for "loose" ("los" in Dutch), "smile" ("smuilen" in Dutch, with a slightly different meaning), "call" ("kallen" is common in many Dutch dialects, meaning "to talk"), "though" (is "doch" in Dutch), "hoast" (in some English dialects = "cough", = "hoesten" in Dutch: maybe the root is not Scandinavian but just West-Germanic). By the way, it is not mentioned in this book but the sound shift from "kerk" (Dutch) to "church" does English have in common with Frisian.
Anyway, I like books of this kind. I can assure my Anglophone friends now with even more conviction that Dutch is for them the most related language though I am afraid that that fact helps Dutch more when they read Old English than Anglophones when they want to learn modern Dutch. The days of Willibrord and Bonifatius are beyond any doubt far behind us...