As spotted by travel site Tnooz, the Mountain View search giant is experimenting with Book buttons in desktop search results that allow users to book and pay for hotel rooms without ever leaving the Google search experience.

Previously, such buttons redirected users to hotel websites.

Hotel booking joins retail and food delivery as markets Google is apparently seeking to monetize more directly, and it’s arguably the company’s most transactionalization move yet. 

As Tnooz’s Sean O’Neill points out:

Expedia Inc and Priceline Group contribute to about 5% of Google advertising income, by some estimates.

Google’s efforts could alienate these advertisers and if they fight back by spending less on paid search ads, Google could lose out if it is unable to translate its in-search booking experience into conversions.

Additionally, as with its retail effort, Google faces the challenge of getting hotels to partner with it. O’Neill observes that online travel service TripAdvisor also has a booking feature that allows its users to reserve and pay for hotel rooms without leaving its website , but adoption appears to be modest.

While Google might have an easier time convincing popular hotel chains to team up with it, prospective partners will have to grapple with the fact that Google transactionalization reduces the control they have over customer experience, something that some of them might be unwilling to deal with, at least initially.

But despite all the risks and challenges associated with transactionalization, Google might have little choice but to continue to expand these efforts.

That’s because the company’s core ad business is increasingly under attack and with ad blocking only gaining momentum, the search giant may have to find other ways to monetize search if it wants to ensure that it can pay its hefty bills years into the future.