Writing this entry in English about a book for learners of English is slightly absurd, so let me be direct about the position it is written from: I work in Spanish and deliver in English when a client needs it, and this is the book I keep at hand for that. Each unit is self-contained and fits on two facing pages. No narrative, no compulsory progression, no chapter that depends on the previous one. It is built to be opened in the middle, which explains four decades in print.
What I took from it
The method the format imposes: grammar gets repaired where it broke. You open the unit when a real sentence came out wrong, not in the order of the contents page. That difference decides whether the book gets used or abandoned around unit twenty, which is where nearly every grammar book gets abandoned.
The units on perfect tenses, where most of the ambiguity lives when writing to a client who does not share your language. The difference between something that finished and something that still has effects is not a matter of style when what you are describing is the state of a delivery.
And one thing it does well that almost nobody does: separating what is wrong from what merely sounds odd. Two different problems, fixed differently.
What I left out
The illusion that it teaches you to speak. It does not: it corrects what you already half use, and anyone who works through it without writing anything of their own will finish the book having changed nothing about their writing. It also carries none of the vocabulary of a trade, which is where a technical conversation actually stalls.