I found an excellent book online at Google books

Trinity and reality; and introduction to the christian worldview - Ralph Allan Smith
- as you may know, most of the books offer only limited previews, but in this case you get the full book.
- I've only read two chapters, but I found them excellent. This is the best treatment of the Trinity I've yet read. (I'll admit I haven't read much on the subject.)

If you haven't tried this service (I might be one the last to do so) I recommend it to you. In all the cases I've checked so far, you at least get the first few chapters and so can get a good idea if you want to purchase the book.

Notes;
1. books.google.com