Yugoslav government was pretty much bullied into joining the Axis. The regent - prince Paul Karadjordjevic - as well as the key ministers were personally pro-British (prince Paul spoke English with his wife and children) but after Yugoslavia was surrounded by Axis countries they saw no way to escape being dragged into the war other than formally signing the pact with Germany.

As for leaving the Axis, Yugoslavia didn't even formally leave it. Hitler was so angry after the coup in Belgrade that he didn't want to listen for the new goverment's assurances that nothing had changed in the relations to Germany and ordered immediate attack.

The only English source I can offer now is Winston Churchil's "THE SECOND WORLD WAR".