I've never heard of a comic adaption. Sounds interesting. As for which books I've read, I've read the Hobbit, The Lord of the Rings, The Silmarillion, and Unfinished Tales. Though I've not read Unfinished Tales in maybe 20-25 years.
As I recall the reason Denothor was driven insane was his using the Palantir and wrestling with Sauron. I forgot where I read this, in an appendix or elsewhere. Doing this drove him mad and aged him, wearing on bit by bit. Often he would see only what Sauron wanted him to see, the hopelessness of victory and sure defeat. I also read, not sure if it is true, that is how Saruman was ensnared into serving Sauron. Though I also read that it was his study of the rings of power that ensnared him into wanting the 1 ring for himself and lead to his making his own ring.
It is also said that afterwards, only someone of strong enough will could really use that Palantir to see what they wanted. Others would see only Denethor's burning hands in the stone.