Somewhat of an oxymoron - speaker stands for the floor for bookshelf speakers.

I sort of agree with ghdprentice. When I set up a pair of 'bookshelves' on stands I used some 24" high steel stands w/4 fillable posts and attached the speakers to the stands using a generous amount of Blue Tac. In order to counterbalance the speaker's weight I filled the stand's posts with steel shot. But, unlike ghdprentice, I only half filled them to keep the weight lower in the post and reduce it's 'tippibility'. BTW that Blue Tac really worked! It was hard to remove the speaker from the stand when you really wanted to. It did not damage the speaker in any way either. 

Depends on the height of your speaker's tweeter (from the floor). Normally it will sound best when it is at ear level. Room and tweeter/speaker design might accommodate a slight deviation. For normal sized monitors we're usually talking about something between 24" and 28".