I have ordered GM service manuals since 1974, and the older versions were in several volumes (and they never passed 20 pounds of weight!). But for the Volt, it will be at least two volumes. One would be for the body work, and maybe engine and service guides using the error codes from the scanners, and the second one would have the electrical schematics, wiring and connector locations, and maybe another list of guides. It varies with each model. My last purchase was the two volumes for my 2009 Chevy Equinox (see my signature), and all my GM vehicles last an average of twenty years each, so I don't buy many sets.
But I am old-school, and I prefer to calmly read a physical book than read many screens on a laptop (I have the PDF versions, too). I do recommend taking time to sit and read every page. Your brain will not absorb everything, but your memory will remember where you will find items that will be important later, such as replacing fluids and parts. Some service procedures will need specific GM tools, and that will help you decide if you will try to DIY or let the dealer do it. That alone will halp you make better decidions and save money when your Volt needs servicing.