The best detailed Paris street map to buy is The Paris Mapguide from Michael Middleditch. I’ve not found a Paris street too small to be included in this book. There are some outlying arrondissements that are not covered fully though, so beware if you’ll be traveling at the outer edge of any of the border arrondissements of Paris.
I wouldn’t recommend this map book for Paris if I didn’t love it myself, and I seriously love this map book. This Paris map book is soft cover, just slightly bigger than paperback size (not thickness, it’s 64 pages), and is extremely detailed. It contains a Metro/RER map, RATP bus routes and shows Metro stations within neighborhoods/quarters. This last feature is not to be overlooked. A Paris Metro map from the RATP is great to have, but if you overlay the metro stops with a Paris street map you’ll have a god-like understanding of how to move about Paris. You’ll be giving locals directions with this map (this has happened to me multiple times, including once where I gave a nice elderly Parisien lady directions near Charles de Gaulle Etoile and just after helping her another nice Paris gentleman stopped and asked me: “You OK?”, not realizing that I was the one giving directions to the local Parisien woman).
Where can you find/buy this Paris street map? That depends on where you live.
The Paris Mapguide at Amazon.com for those living in the United States.
Live in the UK? Buy the Paris Mapguide from Amazon.co.uk
Vous habitez en France? Amazon.fr now carries The Paris Mapguide.
Michael, if you come across this page, I owe you a pint (or a beverage of your choice).