It's pretty obvious to me that they're actively looking for drugs, cash, and child porn. "Looking for explosives" is a transparent excuse for any of this - a proper ETD swab or explosives sniffing canine would pick up real explosives far more efficiently than a TSO manually pawing through any of these items.
In the past, TSO's have flipped through passengers' private papers, as well as books and magazines, and even passports, actually reading the private information contained in some documents, with the stated (ludicrous) justification of searching for 'sheet explosives', which are apparently explosives that have been flattened into paper-like sheets and disguised as printed or written materials. It was a transparent fishing expedition for child porn or any other illegal activities, which TSA is neither empowered to search for or investigate if found.
They have also used the (slightly less ludicrous) justification that they are looking for prohibited items such as blades, hidden within the pages of a book or magazine. Though this excuse might hold water in the case of books or magazines, it does not explain why TSOs read the contents of personal papers. Again, this was a transparent excuse for an illegal fishing expedition.
And of course, they have stated that books present as large opaque masses in an x-ray scanner, necessitating a hand inspection to insure that they are not blocks of C4. Two-and-a-half-pound blocks of C4, however,
don't seem to present a similar opaque mass on the x-ray, at least not in Fayetteville, NC.
I'm not sure what a book actually looks like in one of TSA's x-ray scanners. I wonder if there are any sample images of books on the TSA web site?