Our Mutual Friend is a very important book to Desmond. He's read everything Dickens has ever written barring this last novel, and he's saving it. It's going to be the last thing he reads. At the point of suicide in Live Together, Die Alone, he sets about reading the book so we know how serious his situation is. In the same episode, when he is in stormy seas on his yacht, he takes time out to wrap this precious book up and keep it with him. He gets knocked unconscious and goes overboard and washes up on an Island, and he's still got his book with him! So, you get the idea? It means a LOT.
Cut to Desmond in Orientation, realising the computer can't be fixed after a bullet went through it. Thinking on his feet he takes time out to gather some things before scampering off to his secret yacht and sailing away from the Island. What does he not take with him? Our Mutual Friend.
Good one, Des.
And if you're going to argue that Des just forgot his precious book, well, let me present this particular anomaly. If the claim is he forgot this most important book, let's take a second to review what he remembered to take with him as he bolted out of the Swan Station. The vaccine. That's right. He grabbed vaccine vials and stuffed them in his bag and ran out of the Swan Station. You may be thinking, So what?
I'll tell you what. Because at this moment in time Desmond had already found out there was no reason for quarantine! He'd noticed Inman's ripped suit, followed him outside and breathed the air and discovered Inman's betrayal! Desmond knew the vaccine was a conceit Inman used to keep him locked in the Swan Station, pushing the button, whilst he made his escape! (He can't even claim ignorance - he told Claire it was useless when he arrived back at the Island and saw her inoculating herself and Aaron.)
So Des forgets the book that MEANS EVERYTHING to him, but remembers the vaccine that IS USELESS to him. Good one, Des.
But that's not all. Back to that moment in the Swan Station. The blood draining from Desmond's face as he sees a smoking bullet hole in the side of the computer and realises the timer can't be reset. He has a bash at fixing it, but to no avail. So. You can a) Do what Des did and gather things and then leg it. Or b) You can remember there's a Fail Safe switch to be used in just such an emergency.
Allow me to play devil's advocate on this and suggest that Desmond thought turning the Fail Safe would mean instant death. As such, knowing he had a yacht to escape on, Desmond decided to make a run for it. Fair play, you may think - only, as I will proceed to, Desmond had a couple of days work required on his yacht and he had good reason to think not pushing the button would be genuinely catastrophic.
So here's the clincher. The real pick of the bunch. Locke says to Desmond, "Tomorrow we're going to find out what happens if that button doesn't get pushed." And Desmond goes along with it! Desmond joins Locke in keeping Eko from pushing the button and letting the timer run down until, right towards the end, he realises what he knew all along: that not pushing the button definitely produces a result.
Rather than saying nothing, Desmond's response to Locke ought to have been something like, "Look, box man, I don't care what some video told you, I didn't push that button in time once and the whole bloody island shook! Metal objects in the Swan Station were flying around! It was terrifying, brother, and if you think I am mad enough to let that happen again then you're crazy."
But Des didn't say that. No. He just went along with it. Jack asked him, "Do you ever think that maybe they put you down here to push a button every 100 minutes just to see if you would? That all of this - the computer, the button - it's just a mind game? An experiment?" And Desmond's response was the memorable, "Every single day." Except it should have been more like, "No, because one day I didn't push it and it caused a serious island-sized earthquake."
Good one, Des.