She sends him the message when they're two days out. That should give him time for navigation errors. The atmosphere on board is full of excitement and anticipation - Lux jiggly with the prospect of recovered fortunes, Other Dave and Anita discussing what they might find, Miss Evangelista just full of the adventure. Nobody notices that River's not really focused on the job at hand.
It's been a few years since she last saw him, since she said goodbye to him amid the ruins of an ancient temple and blinked dust out of her eyes as the TARDIS dematerialised. He'd been as outwardly cheerful as ever, promising her they'd bump into each other again in no time.
He was lying, of course. She's learned over the years that the Doctor, for all his charm and energy and intelligence, is an inveterate liar. He lies if he thinks it's the right thing to do, if it's going to ease someone's pain - she found that out the hard way. It took her a while, but she realised eventually Time Lords don't think like human beings.
But even though he was lying, as the door closed on the TARDIS and he disappeared again, she holds no resentment. She lies herself, after all, and it's the way he is, it's the way he's always been. He wouldn't be the Doctor if he was scrupulously honest - in many ways, with his arrogance and his utter certainty and his superiority, he's not exactly a nice person - but River knows she was swept into his orbit from the moment she first met him. That second when he looked into her eyes and promised to save her was the moment her life changed forever.
They land on the Library, and it's silent. Professionalism kicks in: she gives the orders, steers Lux and the vapid Evangelista along, follows Lux's map. There's no sign of him, so she orders a break and, in a quiet corner, gets out her sonic screwdriver. Even though she's had it for years now she still doesn't know all its tricks; but she does know how to scan for alien life forms. It plays up to start with, signalling that there's life all around, but she gives it a firm tap against the wall and tries again, and there it is. The steady beep telling her where he is.
Coming out from her corner, she gets everyone up again, and following the sonic leads the way to the Doctor.
At first she thinks he's playing. He does that - he's done it before, once or twice, but usually there's a wink or a blink or a sign indicating their complicity. There's none of that now. He's in one of his commanding moods, ordering the team around, talking flat out, moving around as if he owns the Library. And not a sign to her. Plenty of comments to the red-haired woman - and she's odd, it's been a while since he had a regular companion - but nothing to her.
And then it finally hits her, as she flicks through the diary, as she meets his eyes. There isn't the weight she's used to, not quite. Behind the guarded veil there's loss, oh yes, and age and knowledge, but none of the sheer power of time itself she's got used to looking at. It took her a while, in the beginning, to grow accustomed to the universe in the Doctor's eyes. Not having it there - and more importantly, not having the knowledge of her in there - it hurts. By the gods, it hurts. What hurts as much is his suspicion of her; she realises now that he's always trusted her because he's always known her, as simple as that. The ease with which he took her hand on that first meeting, that ease must have grown from whatever's going to happen in the coming hours.
They're interrupted by the strange alarm, and he's into action instantly. River closes the diary, lays it down, following his rules. She has a feeling this adventure with the Doctor will be different from any other she's had before.