I think reading books is a good way of practicing concentration. The moment you stop paying attention to the text, the reading falls apart completely. It gives a very direct feedback when your mind has wandered.

You can get away with skimming texts on the Internet, indeed you kind of have to with all the ads, inline links, and other visual cruft clamoring for your attention, but that usually does not work particularly well with books, especially if it's a more demanding read.

Reading can be shockingly hard if you haven't done it in a while, but it rapidly gets a lot easier.