I dunno I am so iffy about a Deathwatch book, because we already have DH, and DH covers all three forms of inquisition. (although it more or less only likes the other two) I think Deathwatch would be better off as a supplement to DH then a stand alone. Unless...... Unless that is of course the Deathwatch are not the players at all, as in they are the enemy. HAH, think of it if you were a team of mercenary Xenos on the run from what seems to be the most terrifying enemy of all, Marines.
Otherwise the only point to it would rather than be an acolyte again would be to play as a marine. Esepcially since we can not play as either chamber militants in DH (although you can get close by taking the soriatas path)
Also to rebuttle a comment above about Inquisitors coming from Deathwatch... No there are no Inquisitors in the Deathwatch. IT was a joke of semantics. Deathwatch is the chamber militant, a tool to be used by an Inquisitor of Ordos Xenos. Eisenhorn was such an Inquisitor and he did not even have the power to summon the Deathwatch. So.... Ordos Xenos needs to have an Inquisitor lord authorize the use of Deathwatch.
Deathwatch comprises Marines from local chapters to the specified engagement zone who show a knack for killing the enemies present. Old fluff stated that they would stage their death and take them in secret. Current fluff now is that they go with honor, serve a few tours of duty and return to their parent chapter and re-don their chapter armor but with a new spiffy death watch shouldar guard as a badge of honor.