"Legend" is a prequel to and the end of the Area 51 series. It fills in several blanks about the series and clarifies some puzzles, but I didn't think it was as good as the preceding nine novels.
I have thoroughly enjoyed this series and am sorry to see it end. All in all these books offer a very unique view of history, religion, and UFO's.
The final book fell a bit short of characterizations and plot but made up for it in filling in the blanks and offering a somewhat bizarre view of religious history. It is required reading if you have read the series, but I wouldn't recommend it as a stand-alone read.