Well....the same rumors indicated that Central and South American sections of the new edition of Motorcycling Adventure Handbook will be much improved. Hope that's the case.
I carried a Word document about 150 pages in length on a yearlong trip, stashed in my netbook. On it were the pasted bits and pieces from ADVrider, Horizons and Lonely Planet websites, divided by country and/or subject. At times it was helpful, but more often it gave me a backup plan, allowing me to proceed in my usual blundering way without fear I'd be left totally high and dry. People with greater organizational skills might do better than I.
I carried the big Lonely Planet books as well: Central America and South America. They were helpful mainly for local maps, which I'd tear out and put in the mapcase on my handlebars. Eventually, I'd tape them back in the book, which is ridiculously messy but perfectly functional. Others seem to prefer GPS for some reason.
I did general advance planning using a single map of Central America and Mexico, plus another of all of South America. All other maps I bought along the way.
James' advice on site searches (above) is the key to fulfillment.