The only vacation time I have available during the year is in December, my question is I want to visit Germany Austria and Switzerland, I am concerned about the weather and the activities available during this month, Is it doable? and if yes what do I need in order to enjoy it, also I need to buy a good guide book for those countries , any advice?
Unless you want to enjoy the winter sports in the Alps, stay in the cities. The climate is usually pretty damp and overcast, and the hours of daylight are greatly reduced. This doesn't limit your activities in the cities, but it reduces the countryside to a series of dull gray tableaus before it darkens again. Most guidebooks that offer suggested itineraries (including Mr. Steves') are written for summer travel in mind. You can get some good ideas from just about any guidebook, just ignore the suggested itineraries unless it's written specifically for the winter.
If you are travel past the first w/end of December you will begin to see the Christmas markets, which are magical in the larger cities. Also the museums are best seen in the winter times. Save the outdoor activities for better weather times! My favorite memory was hearing all the great music around the Christmas season. If you are a classical music fan, try to hear Bach's Weihnachts-Oratorium in one of the great music halls of Germany or Austria! You'll never forget it! :) Best Wishes!