I don’t know of any books that specialise in the teaching of English for insurance. I saw one years ago for teaching maritime insurance which is a very specific area. However, like you I believe that you can teach this without knowledge of the field of insurance. Your students are experts in the field of insurance; you are the expert in English language and the learning process. You can do something very profitable together. This is the real field of teaching English for specific purposes.
You can continue looking for a book but my feeling is that no book would really address the specific needs of your students. Take a look at books on legal English and banking; it’s a long-shot but you might find something related to insurance there.
Alternatively, you can ask your students to provide you with the type of documents that they use regularly. If the contents are confidential, they can blank out certain parts (names, etc). You can ask students what sort of tasks they are usually required to do with the documents they give you. Do they read to extract specific information? Do they read them cover to cover for complete understanding? Do they write these documents? Using these documents, you can design your own materials that practise the tasks they mention.
The tasks that you create can practise the skills of reading for specific information, skim reading. You can also do vocabulary study and activation based on the texts. You can use any type of activity or game you usually do to practise vocabulary. You can identify phrases that are commonly used; e.g. “legalese”.often uses shall, must, will. This can be a springboard for language study. When you’re studying language, the support you use doesn’t need to be focused on insurance.
Using the documents, you can give them out to groups of students. They then prepare questions on the texts that other groups will answer. Ask them to think of questions that they deal with regularly in their work. This gives them practice in asking questions and gives you the chance to get an insight into their work.
This all may seem like hard work. It’s certainly more time-consuming than using a textbook. I don’t see the alternative, unless you get lucky in your search. You can use books on material design and discourse analysis to help you plan the lessons. For designing materials, you can also look at my answer on banking English dated 3 December.
A final tip: keep hold of any materials you produce. You never know when or how you can use them again!