There's the old saw that goes something like "Them's that cain't do, teach." A grain of truth there for certain fields, but by no means generally so. When it comes to art instruction books, as best I can tell, the authors actually seem to be artists.
Then there's the case of sports, where it is said that the best coaches and instructors are usually not the best athletes. The reason offered is that really gifted athletes practice their techniques intuitively, and aren't really aware of the details less gifted people need to learn in order to improve their skills. A lesser athlete has to pay close attention to such details and therefore is better equipped to impart them to others.
So does this apply to art instruction in studios or via books? Maybe, but I'm not sure, never having seriously analyzed the situation. The reason why I haven't is that when I buy an art instruction book (something I seldom do anymore), I gravitate to books where the author's drawings or paintings influence my decision to purchase. That is, if I don't care for the author's artistic style and degree of expertise, I won't buy the book.
Do other aspiring artists do what I do? I have no idea. But let's consider the case of an artist whose books have been in print for the better part of a century, clearly implying broad acceptance.
That would be Harold Speed (1872-1957). A short Wikipedia entry on Speed is here
. Matthew Innis has a long post dealing with Speed here
. Besides many examples of Speed's paintings, Innis includes step-by-step images from a portrait tutorial.
Speed wrote two books about doing art. They are "The Practice and Science of Drawing" and "Oil Painting Techniques and Materials," respective Amazon links here
. Innis mentions that the latter book is in the public domain and can be found online.
So how good was Harold Speed? Let's look at a few of the portraits he painted.
Frank Pomeroy - 1898
Lady Diana Bridgeman
Sir Henry Campbell Bannerman - 1907
Lady in Green
Lillah McCarthy as Jocasta - c.1907
My reaction is that Speed did good, competent work, capturing his subjects with solid skill. Lacking is the flash, dash and artistic personality that more famous portrait painters such as John Singer Sargent, Anders Zorn, Joaquin Sorolla, Giovanni Boldini, Philip de Laszlo and a few other contemporaries and near-contemporaries. I suppose this comes close to the case of sports mentioned above. Whereas the painters just listed might have some students or might have passed along tips regarding painting, so far as I know they never wrote books on the subject. Speed wasn't quite an absolute top-level painter if fame is the criterion, but he was highly competent and able to convey what students of drawing and painting needed (and still need) to know.