There are many good travel books 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. It sells for about $20.
There is tourist office located on Boulevard Miguel de la Madrid. This is the state 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 your sight seeing and itineraries