But broadly speaking, Christianity and Judaism seek to serve the same God, do they not? And is not their God the God of Abraham, Isaac, Moses, and Elijah? And don't they share, generally, much of the Old Testament as sacred scripture?
From what I have seen of the Koran, it is radically different from the Old and New Testaments.
You asked: "But broadly speaking, Christianity and Judaism seek to serve the same God, do they not?" I don't really think they do. What makes you say this? What are the elements of similarity? The Jewish and Christian conceptions of G-d are very very different. Jews fundamentally reject Jesus as Christ--we fundamentally reject the *idea* of a deity in human form. You asked "And is not their God the God of Abraham, Isaac, Moses, and Elijah?". The answer is that, yes, Jews and Christians believe in a link between their G-d and the G-d of the people you mentioned (Jews might traditionally refer to "the G-d of Avraham, Yitzhak v'Ya'akov" in this context). So, of course, do Muslims. And, of course, Muslims also believe in the 'Old Testament' as being sacred scripture. They differ from Jews in that they believe the same of the 'New Testament'; Judaism rejects the 'New Testament' entirely and *never* refers to the Torah as the 'Old Testament'. Judaism also traditionally believe in the Talmud as being sacred -- Torah she'be'al'peh (Torah that is in the mouth, as opposed to written). That adds a collection of more than a million words of Jewish thought that is unique and not at all Christian (or Islamic) in outlook.
I think it is easy both to over-play and to under-play the resemblance of each of the three religions to each other. They are distinct and distinctive, sister faiths. They share some important concepts -- and are utterly unlike each other in other ways.