Few high schools have yet to put them to use, but free digital textbooks keep on coming.

There are now 27 textbooks, partly or completely aligned with state standards, with two more in the wings,  following  completion last week  of the second phase of textbook review by a state agency, the California Learning Resource Network (CLRN).

All but one that went through the review process are math and science textbooks, primarily written by college professors. The exception is  an outline of American history.

It’ll be a few years before digital texts find their way into the classroom, but the first wave is coming and inexorable. And it’s not just because these 27 are free. What’s attractive are digital texts’ flexibility and interactivity. Teachers can mix and match content –  pick a chapter from one book and combine it with  another to meet individual and groups of students’  needs. Soon digital textbooks will incorporate videos and Internet links. And e-text readers – Apple’s iPad, Amazon’s Kindle – will be getting cheaper and more versatile.

Gov. Schwarzenegger gave a big boost to digital texts a year ago with his Free Digital Textbook Initiative – an invitation to publishers to align their texts to state standards. Sixteen textbooks were rated for adherence to standards in the first round; four were updated and 11 new texts were added in the second.

Riverside Unified has taken the lead in California, with at least one class in every high school trying a digital textbook and reader out this year. Charter schools and private schools, unencumbered by lengthy approval processes,  will be the other early adopters. Over the summer, teachers with the charter group Leadership Public Schools will create three variations of each of six science textbooks published by Palo-Alto based CK-12 Foundation, including a version for advanced students, according to CK-12 co-founder Murugan Pal.

CLRN plans to do a survey to districts to see who’s using free digital textbooks. At this point, no-one, not even the publishers,  know.

CK-12 has quickly become a leader in the open-source field. It has added teacher editions to its textbooks and more student activities to go with the curriculum. It plans to embed hundreds of videos produced by Khan Academy’s Salman Khan, a former hedge fund manager and investment banker from Menlo Park whose self-produced instructional videos in math and science have made him an Internet cult figure.

It’s not surprising that districts haven’t adopted digital texts yet, even though they could simply print and hand  out copies of CK-12 Flexbooks  and save thousands of dollars in textbook costs. Schwarzenegger launched his initiative too late for schools to incorporate the texts this year. And it will take at least three or four years for districts to study digital texts, train their teachers in using them and figure out how to make digital readers available to students.

The state or a private foundation could jump-start the process by underwriting the cost of creating teacher workshops for digital textbooks. Professional development by county offices of education for all 30 books could be done for a little more than $500,000, according to CLRN Director Brian Bridges.

The Legislature has frozen textbooks adoption for first through eighth grades for five years. Most districts don’t have money to update textbooks anyway these days.

That’s created opportunities for publishers of open-source digital texts, like CK-12. Teachers with outdated physics books can download CK-12’s chapters on nanotechnology, for example.

Bridges believes digital textbooks will replace printed texts when the next round of textbook adoption resumes, probably around 2017. And the behemoths of  for-profit textbook industry, like Pearson Learning and Houghton-Mifflin, will be in the mix. They’ve learned the lessons of the music industry, Bridges says, and are already embracing, not fighting, digital texts. CK-12 and other electronic publishers are the catalysts behind a paradigm shift in learning.