I've enjoyed Lonely Planet books for years, and Lonely Planet Paris (City Guide) is another great guidebook in the series. It is full of historical background as well as practical tips. I have never been to the city, but I am planning a trip now, and I have been going through this book to prepare. The Arc de Triomphe, for example, starts out with a brief history of the monument, explains some of the plaques, describes some of the sculptures, and concludes with the viewing platform.
WIKIPEDIA VS LONELY PLANET It's all clear and well-written, but in this age of Wikipedia, how necessary is it? If you visit the Wikipedia page for the Arc de Triomphe, you'll find much more historical detail, many photographs of the plaques and so forth that are only briefly described in the guidebook, and better directions about how to gain access to the arch.
I've gathered a lot of Wikipedia entries for the trip and put them into my Evernote account, so I'll have them easily accessible. I also plan to have wireless access, so I doubt I will be consulting this guidebook as much as I would have ten or twenty years ago. Wikipedia is sometimes inaccurate when it comes to history (so is Lonely Planet), but it is usually on target when it comes to more practical matters like gaining access to a site like the Arc de Triomphe, and it has photographs galore.
If you have wireless access, should you bother bringing it along on your trip? I still think so, because it offers a handy way to find quick synopses of sites, and it still contains great tips on food (though Yelp!, like Wikipedia, is also a potentially more useful alternative). Gathering the Wikipedia entries takes a lot of time, and it is definitely easier to thumb through this guide, so it still has a role to play in planning for a trip.