Agile Estimating and Planning
Mike Cohn is an Agile Alliance co-founder, Information Technology executive, and the author of two excellent books: "User Stories Applied" and "Agile Estimating and Planning". His first book became de facto standard for creating user stories. His second book is often described as one of the best practical guides to estimating and planning agile projects. I really like and recommend both of Mike's books.
Estimating and Planning are necessary non-value adding activities on a software project. They are non-value adding because they do not add a direct value to the final product as perceived by the customer. They are necessary because the consequences of not doing estimating and planning are dire: we would end up with no data to support any quality decision-making process on the project. When is the project going to be done? What can we have completed by a certain date? Should we release now or shall we wait another month and release with more features?
You could argue that during estimation we analyze and evolve our requirements, which may be considered a value adding activity. I would agree with you. The trick is to make this process fast and lean by doing just enough analysis and eliminating all unnecessary wastes. You will need to figure out what works best for you and your team, but if you are wondering where to start, start with the "Agile Planning and Estimating" book. You will find simple, pragmatic, logical, common-sense methods and techniques to make your projects successful.