Hi Peter,
I've tried to answer this a few times but I realise I'm basically trying to tell you how to be a BA in a couple of paragraphs.
You need to get your list of function points i.e. you need to scope your system. And the business needs to tell you what that scope is. So I suggest you look at Amazon to get a broad view of functional area and the functions in each area. Remember that you are only seeing the functions available to a visitor and a customer. You can't see any back office stuff which you will need to include in your estimates.
Use that as a starting point and workshop the functionality with your business. Remember to include administration functions, reporting, external interfaces (e.g. accounts system, warehousing system, etc). They must tell you what they want in the system but the work you do on amazon will help make the workshop more productive.
How deep you go in your scoping exercise depends on how much time you have to do it basically.
I don't follow a particular estimating methodology, so there are no books to study, sorry. (note to self: write an eBook!)
Things to think about are:
1. Functional areas and their functions. This leads to your list of function points. You need to provide weightings on these to do the estimates
2. Who are the actors that interact with the system and what functionality will they use.
3. What processes should the system support e.g. ordering / order fulfillment / delivery / tracking; website registration
4. What are the main entities in your system e.g. customer, order, wishlist, book review. What are the possible states of each entity and what functionality causes the transition.
5. What are the business rules - start collecting these
The deeper you go into this, the better your list of function points will be and the better idea of their complexity you will have.
Sorry I can't be more helpful but these forums are not really the place to teach you to be a BA. You should probably look into getting some training and / or finding a mentor.
Good luck. If you have some more specific questions, post away.
Kimbo