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I have used Lonely Planet ever since the first time I travelled back in 2003. My uncle - a travel guru - recommended it to me and I've been a loyal customer ever since.
I know this doesn't really count as a "book" but there's actually quite a bit of reading involved before a trip. I like to know the history of the place, the political status, and all the neat little stories that Lonely Planet puts in the little boxes on random pages. By the time I'm en route, I've typically read about all the different regions and decided on what the best route would be in order to visit all those places. Along with an intense spreadsheet, a Lonely Planet book is basically all I need when planning for a trip.
I was in Nicaragua for a volunteering trip, so it's not like I picked this book off a shelf or spun a globe and pointed with my eyes closed. I knew I was going there and I knew I wanted to stay longer so I could backpack and know the place better. I was still in school so I didn't have much time. So I skipped a week of school. Even then, I didn't have much time. I visited San Juan Del Sur, Granada, and the Ometepe volcanoes (even climbed one!). That is also where I stayed in the cheapest hostel I've ever stay at in the world: a whopping $2.50 USD per night. The beds were all crooked and looked like used military stretchers, and there were about 30 in a room, lined a foot apart. A guy came out of the shower and said a scorpion ran across his foot. Good thing I don't wear my glasses when I shower.
The trip ended across the border in Costa Rica and since I was barely doing much there (only visited Monteverde) I didn't think I needed to buy a Lonely Planet. And as soon as I crossed the border I missed the bus and had to wander around a sketchy town with no map and no clue what to do. I stayed in a hotel that had blood-looking stains on the walls and I swear I heard gun shots at night. That's what happens when you don't get a Lonely Planet.
In conclusion, I highly recommend it :)