You can start with the Wikipedia article (see the first link below) which links to many other works.  The second link gives a more concise summary on the subject.

http://en.wikipedia.org/wiki/Digital_divide

http://whatis.techtarget.com/definition/digital-divide

Hello,

The Pew Research Center published their report "Americans and Their Cell Phones" in 2011 http://www.pewinternet.org/files/old-media/Files/Reports/2011/Cell%20Phones%202011.pdf.  I was surprised by some findings.  For instance, the vast majority of adults and teens from households with incomes of less than $30,000 use the internet.

just add some references to answer it:

Digital divides in the world and its regions: A spatial and multivariate analysis of technological utilization            DOI:10.1016/j.techfore.2013.12.026

Determinants of Firms’ Digital Divide: A Review of Recent Research         DOI:10.1016/j.protcy.2013.12.013

Income disparity and digital divide: The Internet Consumption Model and cross-country empirical research            DOI:10.1016/j.telpol.2012.12.011

Briefly, on the Italian case and the relation d.d.-health:

- Romano M.F., Sardella M.V., Alboni F., Russo L., Mariotti R., Nicastro I., Barletta V., Di Bello V. (2015), Is the Digital Divide an Obstacle to eHealth? An Analysis of the Situation in Europe and in Italy, Telemedicine and eHealth, 21 (1): 2435 doi:10.1089/tmj.2014.0010

- http://www.istat.it/it/archivio/143073 (free download)

- Ingrosso M. (2013), La salute fra media e relazioni di cura, in G. Vicarelli (a cura di), Cura e salute. Prospettive sociologiche, Roma: Carocci.