There are jQuery plugins, like TextExpander, which work well with portions of text wrapped inside a single element (like a ‘<p>’ tag), but when it comes to collapse block elements, the animation is not so smooth.

Here is a very rudimentary (but working) solution for truncating sections of text paragraphs and adding a ‘read more’ button.

<div class="expandable">
    <div class="inner">
        <p>Some text ...</p>
        <p>Some text ...</p>
    </div>
</div>

.expandable .inner {
    overflow: hidden;
}

$('div.expandable > div.inner').each(function(){
	var expadable_height = $(this).height();
	var visible_lines = 42; //the height of the minimum visible lines of text in px
	$(this).css('height', visible_lines); 
	$(this).parent('div.expandable').append('<span class="see-more">See More</span>');
	
	var toggle = 0;
	$(this).siblings('.see-more').click(function(){
		if( toggle == 1 ){
			$(this).siblings('div.inner').animate({height: visible_lines}, 500);
			$(this).text('See More');
			toggle = 0;
		} else {
			$(this).siblings('div.inner').animate({height: expadable_height}, 500);
			$(this).text('See Less');
			toggle = 1;
		}
	});
});

Demo