Third Millennium Ministries has made an excerpt from J.I. Packer's Concise Theology available in PDF. The excerpt is on the doctrine of the Trinity and can be read here. And true to the title, it is concise - only two pages long.
If you are interested in reading more from Packer's Concise Theology: A Guide to Historic Christian Beliefs, you can find out more information about the book or purchase it here.