Sorting through the question of the genre of Luke and Acts (or Luke-Acts), especially in light of Rethinking the Unity of Luke and Acts, by Mikeal Parsons and Richard Pervo.
If Luke intended the gospel of Luke to be a biography by genre, then why didn't he mentioned Jesus' name in the prologue (Luke 1:1-4)? After all, he was using Mark, which opens with mention of the "beginning of the gospel of Jesus Christ," so he had the precedent of a narrative of the life of Jesus that started out with the name of his subject. That would suggest that his replacing of Jesus' name with a comment about composing a "narrative" concerning "the matters that have been fulfilled among us" (Luke 1:1) means that he's intentionally avoiding going for the biography genre.
Parsons and Pervo would counter (43) that it is the prologue of Acts that justifies classifying Luke as a biography. Acts opens with, "I composed the first book, Theophilus, concerning all the things which Jesus began to do and teach, until the day when he instructed his chosen apostles by the Holy Spirit and then was taken up."
The word "began" often gets interpreted idiomatically, so that the phrase comes out something like, "…all that Jesus did and taught from the beginning…" (NRSV). Note that Joseph Fitzmeyer (AB commentary on Acts, 195), though he uses the same wording as the NRSV for his translation, notes also that it's meant to connect the story of Jesus with the story of the church, which I think my translation above (which follows the NIV, though I usually hate their translation) reflects.
All this to say, I'm not sure Acts is trying to identify Luke as a biography, so much as it's suggesting that the work of Jesus is accomplished throughout both Luke and Acts, and Luke simply contains the part until Jesus is taken up into heaven. This can easily be pushed too far, of course. After all, Luke does contain the full extent of the earthly life of Jesus, and little else. So the content of Luke looks an awful lot like a biography. However, if Luke was wishing to write some form of sacred history (I personally think he was following the LXX), it would stand to reason that he would put his break between the part of the story where Jesus was on earth and the part where he was in heaven.
Henry J. Cadbury, in discussion the proper name for what he would call "Luke-Acts," suggested that Ad Theophilum I and Ad Theophilum II might be suitable as parallels to the books of Samuel and Kings (The Making of Luke-Acts, 11). However, he rejects this idea in part because 1-4 Maccabees were not a single work but rather disparate works that were joined together. I would suggest that Luke may have been looking at 1-4 Kings in the LXX as a model of consecutive books dealing with related topics.
I'm also curious that the opening of Acts which describes "the first book" rather than "my first book. In English, we would say the only if we were talking about a series that was unified from the start. If we said "my," however, we would probably be suggesting that the other book stood on its own, and that this one was written later, in addition.
So ultimately, is Acts a "sequel" (Parsons and Pervo, 126) to Luke, or are they actually two parts of the same work? I'm leaning toward the sequel, but at the same time I think Luke would have considered the two books to be roughly the same genre.