There's no denying that e-books are already big business, and market research firm Forrester is now offering some pretty impressive numbers that show just how big it already is, and how much bigger it will get in the next few years. The firm surveyed some 4,000 people and found that while just seven percent of those actually read e-books, they still bought enough of them to translate to $966 million in sales this year -- a number that's projected to grow to $3 billion by 2015. As for the reading habits of that seven percent, Forrester found that they "read the most books and spend the most money on books," and that they read 41 percent of their books in digital form. That doesn't necessarily mean that they use actual e-readers, though -- a full 35 percent apparently do most of their e-book reading on a laptop, followed by 32 percent on a Kindle, 15 percent on an iPhone, 12 percent on a Sony e-reader, and ten percent on a netbook. Interestingly, but perhaps unsurprisingly, Kindle users seem to be the biggest boosters of e-books -- they do 66 percent of all their reading in digital form.