An assured supply of armaments, petrol and foodstuffs from the US was vital to the British war effort, especially in the early days of the Second World War. The route across the north Atlantic, treacherous enough in itself, was made infinitely more so by German U-boats prowling in their wolf packs, ready for the quick kill. Merchant ships, slow and defenceless, were gathered in great convoys and shepherded across the pond by their escort destroyers, frigates and corvettes, offering at least some protection against the unseen enemy. Martin Middlebrook's account of two such convoys encompasses all the danger, drama and sheer awfulness of life - and death - at sea in the Battle of the Atlantic.