Just days after Apple introduced the iPad and opened an e-bookstore, the biggest player in the e-book market, Amazon, changed the way it sold digital titles. Steven P. Jobs shot off an e-mail to the Apple executive who had negotiated deals with the publishers.

“Wow, we have really lit the fuse on a powder keg,” Mr. Jobs wrote in the e-mail dated Jan. 30, 2010, to Eddy Cue, Apple’s senior vice president of Internet software and services.

The e-mail was brought up as evidence during the second half of Mr. Cue’s testimony in a Manhattan courtroom on Monday, where much of the discussion focused on whether Apple intended to help the publishers raise Amazon’s prices.

Mr. Cue testified on Monday that Mr. Jobs’s e-mail was not a memo congratulating him about how Apple’s entry into the e-book market affected Amazon, causing it to switch to a business model called agency pricing, where the publishers, not the retailer, set the price of the books. Mr. Cue said Mr. Jobs was remarking on the company’s ability to “cause ripples” in the e-book industry, which was then largely dominated by Amazon.