Use this opportunity to "fix" the outline. This is important because the final book needs to have a single voice to be effective. I've worked on multiple collaborative texts and have found that the final outcome is improved when a single individual maintains the overall vision. This does mean over-riding individual writer preferences sometimes, but the benefit outweigh this downside.
Having students find samples of good explanations helps them study and also helps the future writers by providing examples of what other students found helpful.
One of the most difficult (but also most effective) writing project I was a part of required that we cite a source for every piece of content. This was very challenging as it required that we think about where "common knowledge" comes from. For students, this is an excellent way to check assumptions and helps them get into the habit of fact checking what they read and write.
One of the most beneficial aspects of this project has been the discussion of what is a valid source and what is not a valid source. One of the most profound changes in information transmission has been that the spread of information is much less regulated than it was in the past. This has wonderful advantages for freedom of information; but also has dire consequences due to the spread of misinformation. Therefore, one of the most important things education does is to train students to be skeptical consumers of information. This is an essential component of digital literacy and should be addressed in all courses.
Lastly, because students are asked to evaluate a variety of sources of information, they are exposed to a variety of approaches to presenting a topic. This is an excellent way to help them learn the content since they are getting this information multiple times and in multiple formats. This is also a great way to set up the writing of the textbook because students are identifying which approaches work best for them; and the writers will use this as their guide.
This was also an excellent opportunity to discuss copyright and Creative Commons with students. This content is not particularly essential content but it is important for a variety of professional fields and if students create online content in the future (which is becoming much more common). Understanding the basics of copyright and the power of the Open movement is a nice addition to the course.
One minor change I would make in the future is to be much clearer on what the hook and conclusion are. Students avoided these sections because they were unclear. My intention was for students to find good examples how how sites introduced/concluded the material; or for students to find good examples of applications to catch readers' interests. The message here is - be very clear about what you have in mind!
Further Reading
In 2016 the Honors Biology class created Samples and Sources Documents for future classes. These web links will supply necessary supporting data for the writers and will demonstrate sample approaches to the material in the outline.
The grading rubric for this assignment should focus on how well students evaluate the sources and the credibility of selected sources. The purpose is to get students to think about reliability of online information and how to present information clearly.
For a faster textbook process - this step can be moved to the end of the process as a "fact checking" step.
Many classes have a list of useful web links for students. These are often built by the instructor and occasionally added to by students. I have been having students review the helpfulness of these links and have found this a useful activity for both both myself and the students. A useful assignment is to build a list of these links which students can vote on, comment on, and add to. This community approach will both help find the best links for each topic and will help build community as students are directly involved in the course content creation. This has some challenges as rating widgets are rare and complex at the moment. I have not yet built this for my class, but have begun a similar project where instructors can rate online learning aids.
One strategy is to build a form where students rate learning resources.