The best starting place to get more familiar with the island is available on the Steamship Authority, the newspaper style Vineyard Visitor publication.  It's a quick guide to the towns, the culture, the beaches and restaurants, limited local news, etc.  It even has a calendar so you can see what's coming up during your stay (nature walks for kids, evening concerts or fireworks, bands and nightclub events, etc.).  Definitely pick one up and enjoy the read as your vacation feels like it starts anyway as soon as the ferry sets sail.

Local tourist maps are available at most retailers in the major towns, at least of Edgartown, Oak Bluffs and Vineyard Haven.  Yes, they advertise for local restaurants and retailers but, in most cases, these are the same places that will be recommended anyway.  Yes, the map is not drawn to scale but the towns are manageable that it's unlikely that you need anything more.  Besides, the map is so convenient and handy that all but the most aggressive planner are probably fine just waiting until they arrive to get the lay of the land. 

Okay, finally, for the die hard researcher, the two best local online sources of information are the Chamber of Commerce website and Martha's Vineyard Online.  Both are, of course, a bit commercial but each has some helpful information particularly for those with special interests such as renting a house for your visit, planning a wedding on the island.  For locals and travelers alike, there are also listings of services varying from florists and haircutters to buying or improving your home on the Vineyard.

Chamber of Commerce site has information organized as Getting Here, Island Info (such as a Martha's Vineyard Island Map, Getting Around, Places to Stay, etc.), Weddings and Meetings and Groups.  They also have information on job searches, membership in the Chamber and information for the press.

Martha's Vineyard Online features tons of advertisements for local hotels, realtors, restaurants, etc.  They have similar information as above if you need more than is offered here on TripAdvisor -- wedding info, vacation rentals, salons and spas, classifieds, etc.

Links to off-site resources

Martha's Vineyard Chamber of Commerce:

Martha's Vineyard Online: