"A blog is a website where entries are made in journal style and displayed in a reverse chronological order." (Wikipedia) It's easy to read the latest posts, but it's not too easy to read everything in chronological order, like in most books.
Google Reader offers a partial solution through a new option that sorts posts from oldest to newest. You can find it for each feed, in "Feed settings". If you also click on "show all", Google Reader shows the posts from the last 30 days.