This may be the move that flings Kindle into the mainstream. If nearly all textbooks wind up on Kindle, then paying $400 for a Kindle would turn into almost a no-brainer decision for college students [...] A single textbook can cost $150 new -- and still maybe $100 or more used [...] If e-book versions cost even 25% less, that's a huge savings ...I'm less excited: given that most college students don't buy a lot of books for pleasure, this $400 device would be used almost exclusively for textbooks: so you'd have to save a lot of money to make up for that $400 expense. Then there's the fact that the Kindle might not be so convenient for classes that involve flipping back and forth between chapters, appendixes and glossaries.
But the real downfall of the Kindle for textbooks is the fact that a lot of college students sell their books back to the store at the end of the semester, recouping as much as 50% of the cost.
A foray into textbooks will not take Kindle to the next level. It probably won't require a significant enough investment by Amazon to hurt shareholder value but it's definitely not something to get excited about.