This is a great basic overview of the various older Germanic languages from a comparative linguistic viewpoint. Its only weakness, I feel, is that its title, with its particular reference to Old English, implies that that language will hold a privileged position — which may or may not be desirable to you, depending on your interests. By singling out Old English from all the other languages in the title, I assumed the author would frequently make comparisons between a given Germanic language and Old English when at all possible. That, however, was not the case and Old English was really just given the same treatment as any of the other languages, both in length and depth of its description, and it was not at all a focal point when examples of shared or unique features were noted throughout the book. The subtitle, "A Survey of the Earliest Germanic Languages," is a much more fitting description.
Of course, there are times when certain non-English comparisons were most appropriate — for example, his frequent recourse to Gothic or Old Norse is to be expected given their antiquity or positions as the only East and North Germanic languages in the book, respectively — but there were many instances where I felt the author ignored pointing out interesting connections between a given language and Old English, instead choosing to make that connection with, say, Old High German or Old Saxon instead.
As my rating demonstrates, however, this hardly detracted from my opinion of the book. It was interesting, informative, engaging and well-written. I'm very interested in comparative and historical linguistics in general, though, so the author not following through with what I felt was an implication that Old English would be the main metric of comparison was not a problem for me. However, if you're a student of English and not particularly interested in Germanic linguistics generally, and are looking for a book focused on Old English and how its cousins are similar to it, this book may be overkill for your needs. Put another way, imagine that this book was actually titled "A Survey of the Earliest Germanic Languages," without any special reference to English. If you'd still be interested in such a book, then you should check this one out.