Back in my past travels (especially before the Internet changes) travel agent was the way to go. It ensured that everything got done and saved you time and frustration.
Today, anybody can successfully book a full trip with the Internet. You can easily research using many different websites. Again, I prefer using Fodors, Frommers, and Lonely Planet to help me decide what history and sites might be interesting. If you want to buy a book, I strongly reccommend Rick Steves. His books are fantastic. He is not bogged down in details or to cursory in his writings. He gives clear and concise information about cities with maps (even hand drawn) with a lot of his own opinion ( and sometimes humor).
Also, I use TripAdvisor to check past guest viewpoints on hotels. You can go to most hotels websites or other directories, however, the opinions can give some great info and even entertainment.
I would urge trying to book the trip yourself from finding flights early - then hotels / trains or rental cars. It is not that difficult and all your agent will do is email and do the same you will. Many times the agent will know less than you do.