Not as helpful as you might hope, considering that this is, to the best of my knowledge, still the only English-language manual of its type (and not a particularly cheap one at that).
Divided into three parts, the first, Textual Criticism, offers—reasonably enough—a general overview of the practice of textual criticism, of which very little will not be obvious to anyone who knows how to read a critical apparatus and/or has ever read the introduction to any critical edition. To those who do and have not, West's usual chaotic style will make things hard to follow, and its primary merit isn't that it explains anything well so much as that it offers a bit of an overview of the shared jargon of the field—with the caveat that West is West, and more than one common term is only available as a less pithy Latin phrase (e.g. lectio difficilior doesn't feature, but praestat difficilior lectio with the corollary utrum in alterum abiturum erat? does).
The second part, Editing a Text, does not provide much in the way of practical advice for editing a text; that was mostly in the first part as well. Instead, it alternates between bitter axe-grinding clearly inspired by specific criticism West has received (much is made of the idea that it's pointless to dismiss an emendation as unnecessary because an emendation that is unnecessary may nevertheless be true, for example) and "advice" so particular as to be not just useless ("The printer should be offered a clear and well-spaced manuscript or typescript on paper of uniform size with the pages numbered continuously throughout. Only one side of the paper should be used.") but outright ridiculous ("Collations should always be in ink. If washable ink is used, beware of rain."). The take-away message is that to be a good textual critic, you must be good at the things at which West fancies himself to be good (like writing introductions in Latin), and it is less important to be good at the things at which he is less good (like palaeography).
The final part, Specimen Passages, offers some examples of texts with critical apparatuses and some commentary on the choices made; the effort is appreciated, but they are not more useful than just opening any critical edition you already have to a random page.
Given the ongoing institutional decline affecting our philology programs that does not, however, come with an attendant decreasing need for high-quality editions of texts, there's a real need for a manual of this sort. This one could be a lot better than it is, but it's better than nothing, I guess. And at least it's not as painful as reading German.
(West doesn't include a bibliography because "textual criticism is not something to be learned by reading as much as possible about it", instead mentioning just three other works in his introduction (one each in French, Italian, and German). He does, however, frequently refer readers to books on myriad ancillary subjects such as palaeography, specific types of transmission errors, watermarks in medieval and Renaissance paper, &c., which he is then forced to stuff into footnotes. After the thirtieth or so such footnote he should have realised he was being an ass.)