Dark Matter: A Thinking Fan's Guide to Philip Pullman was published on 22 September 2004. Response has been very positive from readers and the few reviewers who ever saw a copy. Unfortunately, it fell foul of major changes in the way Damaris books were published and distributed so it had virtually no marketing. You can buy it from Amazon.co.uk for £8.99 or get it from me for just £6.00 through Amazon marketplace!
It's having a new lease of life with an American edition being published at the end of this month. Amazon.co.uk is selling it cheaper than the UK edition, but it doesn't have an index. It has a nice cover though.