David Kadavy, wrote an interesting medium post about how he used an online service to get his book on a Time Square Billboard. He used Blip Billboards which offers a wide selection of advertising billboards all over the USA. They allow you to buy 15 sec spots for pennies on some billboards. Prices varies according to demand using a bidding system. I don’t think this is a cost effective book marketing tactic, but it’s a lot of fun.