I have visited Mussoorie twice and will be doing so again in a month. The only place I considered contacting for lodging this time was the Carlton Plaisance.
Look elsewhere if you are seeking an anonymous, bland business hotel with a perfunctory staff. The Plaisance is a haven, a refuge from India's noise and congestion, set off from Happy Valley Road in a quiet garden, with views of the mountains. Yes, the building is an old manor with some of the quirks of such an historic building, but for all but the least imaginative that is its greatest charm. It is filled with remnants of the Raj and the great era of India's Hill Stations, and to eat dinner in the atmospheric dining room surrounded by photos of generations of the Singh family is to be accorded a very intimate glimpse of Indian history. I myself have only visited in March or early April and one of my fondest memories is sitting in the garden as the morning sun began taking the chill out of the air, sipping tea and eating toast with the most delicious marmalade I have ever enjoyed.
The hotel is removed from the bustle of Mussoorie, but I always enjoyed the short stroll into town. For those who cannot make the walk or are in a hurry, it is only a very inexpensive taxi ride.
If you are very lucky, perhaps Mrs. Anu Singh will be there when you visit. An elegant and cosmopolitan hostess, she respects your privacy and yet is there for you when you are in need of advice or just some good conversation. She also is quick to help you change currency, arrange trains, etc. A telling example of her kindness: A friend and I had the unfortunate luck of reacting unpleasantly to some Dehradun restaurant food, as can happen to Westerners sometimes when visiting India. For the day and a half we were ill, Mrs. Singh would look in on us from time to time and bring us her special kitchari to nurse us back to health.
I cannot recommend the Carlton Plaisance too highly.