IGN produces several series of maps covering the whole of France. For hiking, I always use TOP 25 maps (1:25000 scale).
You can buy a map covering a part of the grid (Grenoble is 3335OT), have them print a specific area on demand (only slightly more expensive and convenient if you want an area that is in between two maps of the regular series) or peruse them for free online (the system had many problems at the beginning and it is still somewhat difficult to get around but it's all there). I don't know about using the site on the go or printing/downloading, I always get paper maps before serious hikes.
There are also two other options that I never used. TOP 100 are less detailed but I believe that they have more touristic information (trails, mountain huts, it's all there on the TOP 25 maps of course but it's so detailed that you sometimes need more effort to figure things out). TOP 75 maps are new and especially designed for hiking. Unlike the other two, they are not available for the whole country but there is one covering the two main mountain ranges around Grenoble. Obviously it's not online but other than that, it could be a good choice for your trip.
There are also apparently some mobile applications (more information in French here and here) allowing you to download these data and use them on a GPS/mobile device but I never tried any of them.