My opinion is not really mainstream on this subject. Many people would disagree with what I say.
I think that people who work in the oil industry and in law use the same structures of English as anybody else. Bearing this in mind, I think that students of English for specific purposes (ESP) need to study the same grammatical structures as other students d and to revise and practise these structures. They will also need to acquire the same skills as other students (in terms of speaking, reading, writing and listening). The difference lies in the vocabulary that is being used, the frequency of a certain structures or the level of formality that is expected. To give an example of frequency of structures, I am thinking the type of language that lawyers use when writing contracts; they will frequently use modal verbs that are rare in every day speech. What I have often found when teaching ESP is that students know the vocabulary that is specific to their field of work but are unable to use it or unable to construct correct phrases.
An approach that I often use in ESP classes is to use texts that students encounter regularly in their work. I either ask students to give me copies of this type of text or look for them on the internet. If using the internet, a good place to start is on the web page of the students’ employer. I then create lessons based around these texts: the lessons can focus on reading and understanding or on language that is common in the texts. You can look for the language that is frequently used and study that. You can use any good grammar practice book for this.
You can look at the following 2 posts where I explain this more fully:viewtopic.php?f=2&t=317viewtopic.php?f=2&t=230
This approach to teaching ESP takes much longer than using a course book; it’s time-consuming for the teacher. But, by using this method, you will be teaching to your students’ needs. Students are usually thrilled by this approach.
Of course, as I said at the beginning, many people disagree with me on this topic. So, I have to say objectively, that you might want to continue seeking the opinion of others. In any case, you have done the right thing in contacting a bookshop for their input on a suitable book to buy.