If you're trying to build one yourself, have a look at the open source Readability code[1]. It was originally developed by Arc90 and is now used by Apple and Mozilla in their browser reader views. The code has been ported to a number of different languages.

I work on a service called Full-Text RSS[2] that used a PHP port of Readability, coupled with site-specific extraction rules[3] to identify and extract article content from each feed item. It then produces a full-text version of the given feed. The idea is you subscribe to the full-text version in whichever feed reader you use and it will give you full-text articles where you had partial content before.

[1] https://github.com/mozilla/readability

[2] https://www.fivefilters.org/full-text-rss/

[3] https://github.com/fivefilters/ftr-site-config