1. The first sentence is correct if you are giving a very abrupt instruction to someone "Ann - reserve the room". If you mean that Ann has already reserved the room, then it should be "Ann reserved the room". 2. The room has been reserved for Ann - although please note that 'reserved by Ann' and 'reserved for Ann' do not necessary mean the same thing. Ann can reserve a room for someone else. 3.The room has been booked for Ann - in this case, Ann will have the use of the room, but this does not indicate who actually made the booking.

Ann reserved the room. (past tense) Ann is reserving the room (present progressive - Ann's reserving it right now) "Ann reserves the room" sounds like your describing what Ann does, generally. For example: Q: Who is Ann? Ans: Ann is my assistant. She reserves the room(s). *The* room has been reserved for Ann. *The* room has been booked for Ann. They are mostly synonymous, but I think "to book" might be more common as a verb -- to describe the action, and "reservation" would be more common to describe a noun. Examples: Ann made the reservation. Ann booked the reservation. The reservation was booked by Ann. The reservation was made by Ann. I took Ann's reservation (request). I booked Ann's reservation (request). I made Ann's reservation (request). See also... There is a reservation for Ann. A booking has been made for Ann. A booking/reservation has been made in the name of Ann. A booking/reservation has been made in Ann's name.

Yes, both are correct. The word "book" is often used as a verb with this meaning.