''Harry Potter and the Half-Blood Prince" hangs on the two great and abiding themes of lasting literature: love and death. What is most admirable about this, J.K. Rowling's sixth book of the seven intended, is her unswerving dedication to these two primal elements, and her affirmation of their central position in human lives.
On the light side -- lightness being slimmer than ever in this darkening series -- there is a great deal of talk about love. The young heroes and heroines of the Potter series are now 16 and 17, and so they ''snog" and ''hook up" with one another, break up, and rejoin, in an almost bewildering romantic dance. There's an impending wedding between Bill Weasley and the ravishing but irritating Fleur Delacour. We hear about the dire effects of unrequited love, of hasty or ill-matched marriages. The Weasley twins, Fred and George, in their now-popular joke shop, Weasleys' Wizard Wheezes, sell Patented Daydream Charms and the ''best range of love potions you'll find anywhere."