The next two books aren't remotely incriminating either, and he's almost ready to chalk up his moment of panic as a huge overreaction.
From the brief descriptions online, the final book in the series is about Sam forcing Lucifer into the pit, and he knows that his Castiel-related issues didn't really start until well after that. He's pretty sure. He hopes.
For a few brief moments, that hope gets to live.
Then he clicks on the book called It's The Great Pumpkin, Sam Winchester—and honestly, what was Chuck smoking when he titled that—and everything comes crashing down.
IT'S THE GREAT PUMPKIN, SAM WINCHESTER It was, for the time being, over. The new day was sunny and bright, and Dean, who had gone for a walk to clear his head of all that had happened, came to a little park near the motel. Lush trees swayed in the warm breeze, and as he sank down onto a nearby bench, he briefly closed his eyes, soaking in the calm. Across the bike path, the towns children played, squealing and laughing as they climbed the jungle gym. They were alive because of him. He and Sam had done the right thing, he was sure of it; yet still he had a heavy feeling in his gut. The seal had been broken. He'd been sitting there all of five minutes when Castiel appeared beside him in a flurry of feathers—invisible though they were—and Dean, without turning, sensed him. He wasn't quite sure when it had happened, but he had, at some point, become attuned to the angel. It was almost as though he sensed something kindred in his presence, some part of them connected. Because of it, he simply knew when Castiel was near; felt it like a wave washing over him. He spoke before he turned. "Let me guess; you're here for the I told you so." "No." "Well, good. 'Cause I'm really not interested." "I am not here to judge you, Dean." "Then why are you here?" Castiel paused, considering the best way to word his reply. Dean, being Dean, interpreted the moment as hesitance, and swiftly cut him off. "Our orders—" "Yeah, you know, I've had about enough of these orders of yours." Castiel looked at him sternly, and spoke again, quickly so as to get the whole sentence out without Dean interrupting. "Our orders were not to stop the summoning of Samhain, they were to do whatever you told us to do." Dean leaned forward, elbows on his knees, and frowned in confusion. He knew that he was right in this, and yet the idea of angels being sent to follow his command was absurd. "Your orders were to follow my orders?" "It was a test. To see how you would perform under... battlefield conditions, you might say." "It was a witch, not the Tet Offensive." Castiel smiled. It was tiny—something that would barely even register on a regular persons face—but on Castiel it somehow became something much bigger. It unsettled something in Dean, put him on the defence. "So I, uh, failed your test, huh? I get it. But you know what? If you were to wave that magic time-traveling wand of yours and we had to do it all over again, I'd make the same call." Castiel didn't respond, and Dean continued, determined to get a rise out of him, reckless as it was. The angel's calm demeanor was too much for him to handle. "'Cause see, I don't know what's gonna happen when these seals are broken. Hell, I don't even know what's gonna happen tomorrow. But what I do know is that this, here? These kids, the swings, the trees, all of it is still here because of my brother and me." "You misunderstand me, Dean," Castiel said, "I'm not like you think. I was praying that you would choose to save the town." Something hopeful unfurled in Dean, at that, and he stubbornly ignored it. He narrowed his eyes at Castiel. "You were?" "These people; they're all my father's creations. They're works of art." He sighed, staring out at the children, at the sunlight slanting through the treetops, mottled shadow scattered over green grass. Castiel saw all of it, and he thought it a thing of beauty. He wished he could make Dean understand that he saw it that way, that he wanted nothing more than to save it. He looked back at Dean, still watching him. "And yet, even though you stopped Samhain, the seal was broken and we are one step closer to hell on Earth, for all creation. Now that's not an expression, Dean. It's literal. You of all people should appreciate what that means." Images of his years in Hell flashed through Dean's mind, and his fingers itched with the sense-memory of razorblades pressing into bloodslicked skin. Pain flickered over his face, and beside him, Castiel took a breath he didn't need, steeling himself for the risk he was about to take. "Can I tell you something if you promise not to tell another soul?" "Okay." "I'm not a... hammer as you say. I have questions. I have doubts," he looked down, knowing that there was no turning back now, and not caring, "I don't know what is right and what is wrong any more, whether you passed or failed here. But in the coming months you will have more decisions to make. I don't envy the weight that is on your shoulders, Dean. I truly don't." They shared a look, then, and something passed between them. It seemed to somehow signal the beginning of something; something important. Dean felt it, deep. He shook it loose and looked out over the park, certain that he was imagining it. The creature—and he had to remind himself, again, that Castiel was indeed not human—may have smiled, but he couldn't turn into a friend. Dean was sure of it. He didn't get friends; certainly not ones with superpowers. When he looked back, the angel was gone. He couldn't help but feel a little disappointed.
Dean remembers that conversation. Remembers replaying it over in his head for weeks. For a long time, he hadn't been sure that it was even true—was convinced that it was all a ploy, a way to trick him into opening up in return.
He'd had no idea of the kind of risk Castiel had taken in telling him, nor the level of trust that he had already placed in him, even then.
He wonders if Castiel ever regretted it.