Well it is great, but it's not the actual liquid I'm thinking about here but a rather nifty little booklet I found on Sunday in the Bricklayers Arms in Putney.
It is a Camra publication and I wish I had this when I first started drinking beer, it's clear and easy to read. One of the first of it's kind I have read that advertises beer without slagging of other alcoholic beverages. It explains about handpumps and real beer as per Camra guidelines with a light section on beer ingredients which is informative but not boring and then the bulk of the booklet is about the different beer styles. Brilliant piece of work.
And even better, it is free where pubs have them on display so you have something to read when you are sitting there with your drink. Thumbs up!