Numerous excellent travel books have been written about Mexico and Manzanillo in particular. The Lonely Planet Mexico travel book is the authority on Mexican travel in general. This book should be available at any major book or travel book store and online. It sells for about $20.
A tourist office is located on Boulevard Miguel de la Madrid. This is the offical tourist office for the state of Colima and they have lots of information for not only Manzanillo but also the entire state. They provide free maps and can help you plan with sight seeing and itineraries.