Again (as is my default), it's up to what kind of adventure you'd like to have. Let's break this down, like the group or solo adventure (because let's face it: bringing a book along with you is kind of like traveling with other people too....except it doesn't talk back or get sick on MSG).
With text-in-hand, you can readily prepare for your adventure. You can plot your adventure by using a reference, making sure you hit every point you'd like to hit. This can really save on time, if you're pressed for time (which most people in our Western Culture make themselves). You can eliminate the duds before you even reach your destination and focus on the gems.
Reading some books before you get there can save you money. You can find out ahead of time where the cheaper places to eat and stay are, and where are some of the avoidable adventures along the way ("make sure to charter with Captain Ron - he takes the same path as Captain Sue for 1/4 the cost....and he brings oranges").
I'd also like to suggest that bringing a book(s) about your destination keeps you safer. You can learn what parts of town to avoid, what time you should head inside (if you're going somewhere wild), and some of the local laws you might not be aware of. 99% of people don't like to incorporate jail-time into their vacation.
Lastly, if you're going somewhere foreign, it doesn't hurt to study a bit of language before you get there. You'll find that locals are greatly appreciative of your further interest in their culture, and it just make your journey a bit easier.
Adventure. Without a map, we're given to the whims of the wind. Without text, you're forced to problem-solve on the fly, to discover your adventure moment-to-moment. This is exciting, but comes with a great level of risk - you don't have the safety net of knowledge of those who have come before you. Like I've hinted, this can be exhilarating as you talk to locals to find good gems, and you have a sense of empowerment as you discover them on your own.
Feel free to do this - it can be fun. Book your flight somewhere and let the local winds blow you. If you're a low-stress person with a healthy amount of free time for your adventure, then this method could work for you.
In My Personal View.....
I like researching destinations. Not because I like planning minute-to-minute, but I like having a general idea of what's available to do along the way and at the destination - so I can pick and choose. And it has also been my experience that no book, local, or website has ever covered everything there is to see and do - there are always surprising gems.
Simply run a Google Search for your destination, and you'll come up with plenty of resources. I recommend taking an actual book on your trip so you can make notes and reference it. You might not always have internet access on your iPhone; and I'm a traditionalist, there's something about crinkled and marked up books that make me smile.
Ive really enjoyed Fodor's to find really interesting places that are a little more expensive, but it's a good write-up.
Lonely Planet is a solid fall-back I like to read A LOT. And they update every year on their destinations, so the information is always good.
Rick Steves is a great guy for European Travel, and REALLY knows his stuff.
As an aside, I recently used this book for a US roadtrip. GREAT book!
Enjoy, and happy travels!