Libraries are part of a balancing act between spreading knowledge and keeping incentives for writing books high enough that people actually do it.
Libraries purchase books, increasing payment to authors
Libraries have a limited quantity of books and check them out for a limited time, making them inconvenient compared to purchasing a book. For very popular books this means there's a high incentive for people to buy rather than wait to be able to check them out.
Libraries carry a limited catalogue, particularly for highly technical books. This means that for very niche and valuable books the market allows books to be sold at the higher prices necessary to sustain incentives.
Digital libraries destroy this balance. The content of books is available to everyone, instantly, at maximum convenience.
Libraries purchase books, increasing payment to authors
Libraries have a limited quantity of books and check them out for a limited time, making them inconvenient compared to purchasing a book. For very popular books this means there's a high incentive for people to buy rather than wait to be able to check them out.
Libraries carry a limited catalogue, particularly for highly technical books. This means that for very niche and valuable books the market allows books to be sold at the higher prices necessary to sustain incentives.
Digital libraries destroy this balance. The content of books is available to everyone, instantly, at maximum convenience.