Led by sales in its electronics and general merchandise segment (home to Kindle), total revenue at Amazon jumped 51% in the second quarter ended June 30, to $9.91 billion, though increased investments lowered net income to $191 million from $207 million in last year’s second quarter. Worldwide sales in the egm segment rose 69%, to $5.89 billion, with Amazon reporting that Kindle sales growth accelerated in the second quarter compared to a year ago. Worldwide media sales increased at a more moderate but still strong pace of 27%, to $3.66 million. Media is home to print and e-book sales. Total worldwide sales benefitted from exchange rates and excluding currency fluctuations sales would have increased 44%.

Amazon’s slowest growing segment, at 20%, was its North American media unit where revenue rose to $1.58 billion. Amazon said it now has 950,000 new e-book titles in the Kindle Store with 800,000 priced at $9.99 or less.

Amazon CFO Tom Szkutak said growth in the period was the fastest quarterly gain in 10 years and the increased demand means that Amazon needs to continue to expand its capacity in terms of adding more physical fulfillment facilities as well as expanding infrastructure capacity. The company has announced plans to open 15 new fulfillment centers this year and will have more announcements before the end of the year. Amazon also continues to invest to support the conversion from physical to digital products.

Szkutak had little to say on collecting sales tax, noting that Aamzon supports a simplified federal sales tax solution and noted that it does collect sales tax in about half its businesses around the world, a statistic the e-tailer has pointed out before. Amazon had no comment on whether it expects to drop more affiliates in response to states imposing online sales tax collection. Third party sellers accounted for 36% of paid unit sales worldwide in the quarter.

Amazon sees sales slowing only slightly in the third quarter with growth expected to be in the 36% to 47% range. For the first half of 2011, revenue was up 44%, to $19.8 billion and net income fell to $391 million from $505 million. Amazon now has over 144 million active customers worldwide.