Priceline's (PCLN) decision to buy the Amsterdam-based website has proved to be the most lucrative acquisition in the history of online travel. It's outdone Internet mogul Barry Diller's acquisition of travel websites Hotels.com and Expedia.com in 2003. Eight years later, more than 60% of Priceline's bookings come from Europe, mostly via Booking.com. Priceline boasts revenue for its most recent four quarters ended Q1 of $5.53 billion vs. $4.23 billion for Diller's Expedia (EXPE), its nearest rival. Priceline is set to report Q2 results on Aug. 8. "Booking.com is pretty obviously the most important single business within Priceline," said S&P Capital IQ equity analyst Scott Kessler. Priceline doesn't break out Booking.com revenue. But Kessler says that based on geographic data disclosed by Priceline, about half of Priceline's revenue is funneled through Booking.com's home country of the Netherlands. This, he says, gives a pretty good idea of the unit's importance. Get the full story at Investors.com Read also "Morgan Stanley Ups to Buy on Booking.com" at Barron's and "Expedia admits that TripAdvisor and Booking.com are hurting it hard" at Tnooz