Is there a particular reason why Nietzsche chose the word genealogy as opposed to history for his book 'the genealogy of morality'? Is it simply Nietsche literary style or is their a deeper reason?
I would argue that Nietzsche certainly had a purpose in choosing "genealogy" over "history."
Let's examine the meaning of each word (from Google):
History: the study of past events, particularly in human affairs; the past considered as a whole; etc.
Genealogy: The study and tracing of lines of descent or development
We can see from this is that history deals with events, while genealogy deals with lines of development (it would be reasonable to claim from this that genealogy is a branch of history). This carries the corollary that history does not have to assert any connection between what it talks about, while genealogy by definition seeks to make some sort of developmental or evolutionary claim regarding how certain things are related.
The latter is precisely what Nietzsche does in the Genealogy of Morals. He isn't simply listing off a bunch of facts about what morality was here and what it happened to be there. Rather, he's making a claim that he can trace a line through the development of morality itself.
He begins with his idea of "Good vs. Bad" as it manifested itself in the society of nobility and common people. He then argues for how this evolved into "Good vs. Evil" through the "priestly" people (he credits the Jewish people) and their daring "inversion of morals."
He also makes claims, often using his linguistic expertise, on how guilt developed in the debtor-creditor relationship and how this ultimately grew - and was inverted to - guilt in the prime creditor (God) toward the debtors (man).
This is the sort of thing Nietzsche talks about in the Genealogy: lines marking the development of human morality through the course of civilization. It's because he argues that this is a decidedly causal and evolutionary process that he chooses to call it a "genealogy" and not simply a "history," which is much less specific in its meaning.