Total book sales fell 2.5% in 2011, to $27.2 billion, according to estimates released Wednesday by BookStats, the annual industry sales survey coordinated by the Association of American Publishers and Book Industry Study Group. Unit sales rose, however, up 3.4%, to 2.77 billion with the discrepancy due to higher sales of lower-priced e-books.

The increase in the sale of e-books was the major story in 2011. In the trade segment, sales of e-books rose to $2.07 billion from $869 million as units increased 210% to 388 million. With the growth in e-book sales offsetting declines in print, total sales in the trade segment inched ahead, up 0.5%, to $13.97 billion last year. E-books accounted for 15% of all trade sales in 2011, up from 6% in 2010.

The report shows the extent of the impact e-books had on the adult fiction segment last year. According to BookStats, e-book sales in adult fiction increased 117% to $1.27 billion, representing 30% of segment sales, and making e-books the largest single format within adult fiction, surpassing the hardcover, trade paperback and mass market paperback formats. The sales penetration of e-books in the other trade segments, however, was less dramatic and the print segments generated $11.1 billion in trade sales last year.

The fastest-growing trade segment in 2011 was children’s/young adult where sales rose 12%, to $2.78 billion.

The impact of e-books on where trade books are bought is seen in the 35% growth experienced by online retailers in 2011, with sales to online retailers hitting more than $5 billion last year. Sales through all bricks-and-mortar stores fell 12.6% last year for trade publishers, to $8.59 billion. Sales to institutional accounts--libraries, business, government, schools--represented 20% of sales, $5.39 billion. BookStats also reported that direct-to-consumers sales for trade houses rose about 57% last year to $1.1 billion.

This year’s BookStats report is based on over $16 billion in sales collected directly from reporting publishers. The full report is available from the AAP or BISG.