I'm looking for an electronic edition of an older book. It was originally published in the early 1970s, and it appears at multiple reputable bookstores (Kobo, Amazon, and others). However, I'm hesitant in buying it because the reputability of a source seems to be reliant upon the copyright holder being vigilant. For current books, the author is probably vigilant. For older books, I'm somewhat skeptical.

Adding to the complexity of figuring this out: it is a non-fiction work, so it is not a prime candidate for republication. It is also a specialist text, which means that the potential market is small. It seems to have been a moderately popular book in its era, but it was by no means a best seller. It's interest to current readers is purely historical.

Does anyone have any advice on determining whether such a book is legitimate, or if someone is just taking advantage of it's semi-popularity and semi-obscurity?

(Specifics: "The Psychology of Computer Programming" by Gerald Weinberg. Even though I'd like to know for this particular title, general advice is welcome since I'm interested in computer books from this era.)