Developing REST API


I am a newbie to rails and i have a question about developing an API.

i have a product xml-feed with a title, price, category and link, wich
are use by affiliates.

1)The idea is that the affiliates can decide in how they want to
present the feed on their website based on their own conditions.

1)Show all the products below 10,- euro
2)Show all the products higher than 10,- euro
3)Show all products in category X

Can someone give me a direction with some links, books ect where i can
find documentation to develop this idea…