It is unusual to find a dictionary of any sort written by a single individual; nevertheless, Alan Cairns has done the prodigious amount of work to publish this dictionary of theological terms. The content is from a conservative and Reformed perspective. As such, it is a very useful tool.
However, I would change one thing about the book - its title. It should more properly be called a commentary on theological terms rather than a dictionary. This is so because Cairns regularly argues for his positions and against other theological positions. Obviously, he is free to do so, but this is typically outside of the scope of a dictionary. A dictionary typically seeks to give only a description or definition of a term, leaving evaluation to the reader.