Hi,
It's normal that the description of the category is displayed in full on that page, because that's the main page of the category.
The read more button will be used on the listings of the categories for each category of the listing when you activate the display of their description, but not for the parent category description.
If you want to hide some content with a button which can make appear the rest of the content, then here is a page talking about several solutions for that:
www.gavick.com/forums/creativity-joomla3...w-hide-content-38758
Basically, you can either add some HTML/Javascript to the descriptions to handle that, or have a third party content extension with some tags in the descriptions to do that for you.