In reply to crazy pierre: You don't really need a topo for the Cromlech boulders, just turn up and have a go. You're more than likely to find a knowledgeable local who'll be happy to let you read their guidebook or point you to some classics.
If you're really keen to get a guidebook the "Boulder Britain" book has several pages dedicated to the Llanberis Pass - you can get this at the shop in Llanberis. It will also be useful for other areas in N. Wales and indeed the whole of the British Isles!
