By now the answers should be obvious.  Where?  What would you say in the ad?  Who reads the magazines in which you can afford ads?  How many order books on the basis of ads?  Maybe your experience will be better than mine, but I tried advertising a book in Writer's Digest.  My name is fairly well known, especially to readers of that magazine, where I had had a poetry column for 20 years.  The ad cost over $80 (one column, 2") and brought in about two dozen orders, by no means enough to pay for the ad, let alone the books.  This had nothing to do with the quality of the poetry:  people who read the ad can't judge that.  Advertising is just not the way to sell most books – especially books of poetry.