When you write to commissions you don't get to choose the subject you write about. Sometimes it can be something you've never been interested in. Instead of turning work down (and if you do, be prepared never to hear from that customer again), do your research.
Always ask the customer/editor whether there's a particular angle or aspect that they want you to focus on. You need to be enthusiastic otherwise your writing will be dull and won't hold the reader's attention. Speaking to people who are knowledgeable or specialise in the subject is always a good idea if you can. Hopefully, their interest will rub off on you, and if all else fails, they should be able to provide you with some good quotes. 
I've had to write about bricks, car engines and the merits of brussels sprouts before now–none of them subjects that I knew or cared much about beforehand. Whatever the subject, be it telephone books or drain covers, there is always something interesting or unexpected about it.  It's up to you to find it!