It's great news for Kindle owners that they can finally get library books on their devices.

I always thought this was one of the biggest shortcomings of Amazon.com's device. It also highlighted the fact that Kindles are designed as much for buying books as for reading them.

But, while good for Kindle users, it may not be such a great deal for everyone else using public libraries.

I'll bet that last week's announcement that libraries across the country are working with Amazon to offer e-books for borrowing will come to be seen as a turning point, when libraries accelerated their shift toward digital content bound in content-protection software.

The convenience of digital books is compelling, especially to public libraries struggling to manage costs, grow their collections and stay relevant.

At the same time, there are trade-offs that may be overlooked or downplayed as libraries rush to embrace new formats and satisfy the demands of gadget-toting patrons.

For starters, this transformation may erode the democratic nature of libraries.

To meet growing demand from owners of Kindles and other reading gadgets, libraries are shifting more of their budgets from physical books anyone can read to digital copies that require a computer or e-book to consume.

The King County Library System is working on its 2012 budget and expects to dramatically increase its spending on digital copies after digital circulation increased by 150 percent over the past year, according to Director Bill Ptacek.

It now spends about $800,000 of its $14 million material budget on digital and audio books.

"It's a delicate balance," Ptacek said. "We want to have a big enough collection and offering that people who do have the devices will come to the library. On the other hand, we don't want to go so far overboard."

This balancing act is tricky in part because Amazon -- the leading e-book company -- doesn't disclose how many Kindles it has sold. Libraries are constantly asked for Kindle material, but nobody knows the size of this audience.

Ptacek estimates 10 to 20 percent of its 900,000 cardholders have e-reader devices.

Seattle's library system has seen digital circulation double every year since it began working in 2005 with OverDrive, a Cleveland company that runs the digital lending websites of most U.S. libraries.

Last week, OverDrive added Kindle to the list of devices supported by its service.

Libraries don't have to buy special Kindle editions of digital books. They just buy a digital copy from OverDrive, which serves the copy in whatever format the patron chooses at checkout.

Amazon's arrangement also adds a new layer of commercialism into the public service that libraries provide.

Unlike digital books offered in other formats through library websites, Kindle versions require you to complete the checkout process at Amazon's website. The process ends with a pitch from Amazon to buy more books, and the system feeds Amazon's database of customer interests.

It's still early days for digital books. The next step will be applications that let library patrons borrow digital books directly from their e-reader, Web tablet or smartphone. This will appear on a Sony reader coming in October, and could be on the new color tablets that Amazon's expected to unveil Wednesday.

"There is a road map where we're going to be able to do more of the experience within the app," said David Burleigh, OverDrive director marketing.

At the Seattle library, digital consumption reached a "critical mass" in 2010 with the proliferation of e-readers, smartphones and tablets, said Kirk Blankenship, electronic-resources librarian.

Blankenship expects circulation of downloadable books to triple this year from 100,000 to 300,000 checkouts. Overall circulation has been steady at about 11 million.

That doesn't necessarily mean there's been a major shift in reading habits. Budget cuts forced Seattle to dramatically cut library hours, reducing access to printed books and skewing circulation patterns.

Blankenship and Ptacek both see digital copies as additions to the printed collection, rather than as a replacement. But they are having to make decisions about where to spend their limited budgets.

What will the mix looks like two or three years from now? "We'll have a much more robust e-book environment and alongside that we'll have the print collection we'll be doing just as well," Blankenship said. "When you get a little beyond that ... that's much more of a gray area."

In the meantime, I'd argue that libraries should be pushing for ways to share the gains that e-book companies are seeing.

For instance, Amazon pays commissions to websites that refer shoppers to its online store. Why doesn't this "affiliate" program extend to the 11,000 public and school libraries now channeling book lovers to Amazon.com.

The elephant in the room, though, is the tax question.

Amazon is not only the leading e-book company, it's also become the nation's most notorious evader of local sales-tax collections.

While it's fighting to avoid local taxes across the country, tax-funded libraries are going to extraordinary lengths and paying a premium for content to satisfy Amazon customers.

These public institutions are making the Kindle more appealing, and helping to usher in a transformation in which Amazon may be the largest beneficiary.

Maybe it's time to pay the fees.