Think many differences in 2011 versus 2013 RS Paris guidebook?

Try going to the Graffiti wall and look at the section of critques/feedback of Rick's Guide books. Significant changes might be posted there.

The only changes likely are things like opening hours/days for sights, increase $$ for hotels/restaurants/sights, etc. These are easy to find current on the websites. What to see and do won't have changed much (if at all). He may have added/deleted a restaurant or hotel or two but not that likely. I have always used tour books 2 or 3 years old (or more), just looked up current hours/prices. Never had any major changes.

Personally, I'd buy a new book.

I have a couple of RS books, I never buy new ones.. no point in it for me at all.. the things that change, like restaurant and hotel recommendations and prices of such and admission prices,, I never use those features anyways. The location of a site, the history behind a site, that stuff doesn't change so thats what I read about..
I always double check hours of admission and costs online just before we go, they can change or be seasonal. I don't decide whether or not to see a sight based on cost really, so whether its gone up 1 euro or 5 euros I would still visit a sight I wanted to see.

I'm with Pat, not much will have changed since 2011. But a good idea to check websites for stuff you really care about. That said, for our upcoming trip I bought the newest RS books for my Kindle to save weight. Having trouble finding things though, harder than flipping through pages.

I, too, get the latest version via Kindle, but I use sections from the older books for the walks & maps. It is easier to follow in print than Kindle, but you will have the Kindle version for the latest phone numbers, hours of operation and cost. If I don't have an older book, I get one from a used book store. I feel less guilty about cutting up an old book into sections than a new one.

The 2011 edition is based on research performed in 2010. So compared to the 2013 edition, there will be more out-of-date times, prices, and other information. Things do change. If buying the most recent saves you from a single false start or disappointment, it is worth it.