Usually these days I put together a "trip book" with articles and information from a lot of sources, but I also usually take along one full-scale guidebook as well.
I'd be hard-put to make a recommendation for Portugal, because for that one I decided to overcome my feelings and give Rick Steves a try...and it was nearly useless. No special information, a smug tone, and maps that were too sketchy to be useful.
In general, I've liked the Frommer guides (and I'm excited that the new ones will veer a bit back toward budget travelers) for their fairly comprehensive view. Rough Guides are usually among the best for their historical and cultural context sections at the end.
Suggestion: Go to a bookstore with a big travel section, and look through the guides for a place or two you're familiar with, and find the one or two you'd recommend to someone. Once you've done that, get one from that series for your new destination. No guarantees...but at least you'll know the tone and sections...