He got his haircut, suit cleaned, didn't order any more laudanum, left money for his room, and gave his pain relieving pillow away, so Book clearly was planning to die that day.
If it wasn't for the cowardly bartender shooting him in the back (something that no one could foresee), what was Book's plan if he survived the bar shootout?
SPOILERS
Its a great question.
He probably figured in a "one on three" gunfight -- and at his age, and in his weakened condition -- one of the three was bond to kill him.
But his instincts kicked in and he killed all three of them.
But not before one of them at least (Boone) and maybe two(did O'Brien's shot hit?) wounded Books.
So even if there had been no bartender interference, Books likely would have died of cancer AND wounds sooner rather than later.
But that's not the point here. Books set off clearly understanding that he would probably get killed. Perhaps his back up plan if he killed the three guys with no bartender interference was -- he would have just gone home retrieved his money, ordered more laudanum, bought a new pillow -- and challenged three more varmints to a gunfight.