DISCLAIMER: Not a boy, not brunette, younger than him therefore not MK. I own nothing but the idea. It kind of came to me while reading chapter twelve of "Results of a poker night" by White Angel of Auralon. Go read it, it's awesome
It's a rule of thumb that shinobi have to be inventive if they want to survive in the wild. Everything can and will be used as a weapon. Disguise your weapons so the enemy will not see them. For example, there is more to Icha Icha than meet the eye...
Everyone who meets Kakashi always assumes that he's a perverted, simply because they see the orange porn novel with a big "18+" label over it. But no one wondered why there was a 18+ label here.
You see, eighteen is a number associated with the age one becomes an official adult (excluding the time they receive their hitai-ate) and is able to let loose his most... perverted... fantasies become true with the help of other men and women. And that's why Icha Icha is considered a porn novel.
But, if one would pay a closer look they would see that every page of the book is actually highly dangerous, ultra explosive exploding tags. Kakashi keeps one on hand at all times. After all, who would suspect his little, precious book?
The supposedly smutty novels are quite expensive, too. After all, these explosive tags don't grow in trees.
But Jiraiya has to keep up appearances, so not every enemy-nin would be able to purchase an insta-doom book. So on the back of every tag is a little passage of what goes through the mind of a perverted Sannin. "Voluptuous Hips", "Luscious lips", "Hair that makes the sun pale in comparison" and "generous watermelons".
Naruto understood too, once he knew what the book was really. And secretly, he enjoyed the storyline, just not the more mature scenes. How else would he know the names of the characters in Icha Icha Tactics? It's a weakness of Kakashi if nothing else.
And even if some people doubted how Jiraiya got the title of Sannin, even they couldn't deny the amount of damage a single note from that book could deal.
The enemy would be none the wiser. And no one lived to tell the tale if they had seen the hidden weapon in the smut book.