how can i delete me purchase history

You can't delete any item form your purchase history, all you can do is hide items via your computer's iTunes (if using a Mac with Yosemite or Mavericks on it then ibooks are hidden via its iBooks app) so that they don't show in the cloud for redownloading : iTunes Store: Hiding and unhiding purchases - Apple Support

For apps, on the current version of iTunes there is a bug which means that the 'x' on the Purchased link doesn't show for hiding them - clicking on their top left corner (where the 'x' should be) should still hide them.