Samui is well covered by the general Thailand guides. Purchasing a specific map or guide really isn't necessary since there is such an abundance of free guides available. There were at least four free map/guides to the island the last time we visited, plus a free dining guide, spa guide and accommodation guide. If you buy a map or guide before you arrive, you're probably going to feel like you wasted your money.
If you're planning a primarily surf and sea vacation, you may want to consider one of the many guides to Thailand's beaches and islands.