Book excerpt: This is a book about the simple pleasure of pebble spotting. Clarence Ellis is a charming, knowledgeable and witty guide to everything you didn't know there was to know about pebbles. He ruminates on what a pebble actually is, before showing us how they are formed, advising on the best pebble-spotting grounds in the UK, helping to identify individual stones, and giving tips on the necessary kit. You'll know your chert from your schist, your onyx from your agate, and will be on your guard for artificial intruders before you know it. Understanding the humble pebble makes a trip to the beach, lake-side or river bank simply that little bit more fascinating.