What advice should you give? You might explain that while the amount to be learned appears voluminous, it's not infinite. Almost any good book, video course, or ground school can adequately prepare someone for the private pilot knowledge test. If your students plan to purchase books on their own and self-study for the knowledge test, then help them identify the appropriate sections of that material to learn. In other words, if your student buys a FAR/AIM, you don't want him or her to spend any time reading Part 121 regarding airline regulations. It will do little good if the student can't interpret light signals from the tower but knows everything about crew duty limitations on a Boeing 747.
You'll also want to make it clear that they aren't expected to learn everything at once. They only need to know what they need to know for their given level of training. For instance, they don't need to know how to plot a cross-country flight if they haven't soloed yet.
Finally, keep in mind that book knowledge is as important as flying skills. In the looming face of so much information, some students might be tempted to forgo their ground study (and put off taking the knowledge test) while working solely to acquire flying skills. In the long run, this won't help them to acquire that pilot certificate.