I have read that in the Messianic Era Purim will be the only holiday celebrated. What are the reasons for this? Why is the Book of Esther more important than other books, such as Daniel or Isaiah?? And why would Purim be more important than Shavuot?

The source in the ancient midrash (Midrash Mishlei 9, 2) says that in the end of days, all of the holidays will cease except for Purim (and R. Elazar says also Yom Kippur). As with many midrashim, it’s unclear whether this is meant to be taken literally, but the message is what’s important. As you know, Esther is unique in that it’s the only book in the Tanach without God’s name! God made the miracle and salvation there from “behind the scenes”. As the last book canonized into the Bible, God stopped doing super-natural miracles, already 2,400 years ago, enabling man to have more free-will. Just as a parent gradually teaches his child independence, that’s how God treated mankind. Like children, primitive man needed super-natural miracles and guidance and their free-will had to be relatively supervised and limited. Over history, mankind has matured, and God can hide Himself more (although He secretly always supervises), and grant us more and more independence. Our rabbis are teaching us that in the end of days, the Purim type of miracle will be that which we celebrate (and not the super-natural type like Pesach and Shavuot), for we will ourselves be Godly and responsible enough, and mature enough to remember to recognize and thank God, even if he isn’t as apparent as during the olden days. As we enter the messianic era, with the return of Israel and the Ingathering of the exiles, it is already important that we ourselves act Godly, don’t rely and wait for super-natural miracles, and remember to thank God for helping us help ourselves, through the Balfours, revival of Hebrew and the desert, Entebbe, Aliya of Soviet Jewry, and celebrating religiously Israel Indepence Day, commemorating the War of ’48, and Yom Y’rushalayim, for the Six Day War!