Wikibooks, previously called Wikimedia Free Textbook Project and Wikimedia-Textbooks, is a wiki for the creation of free content books. Wikibooks is a Wikimedia project that was started on July 10, 2003 with the mission to create a free collection of open-content textbooks that anyone can edit. Since its founding, volunteers have written over 35,000 modules in a multitude of textbooks.
What is the difference between Wikibooks and Wikiversity? 
Wikibooks hosts textbooks. Wikiversity does not host textbooks. Wikiversity started at Wikibooks, but later evolved into separate project. Wikiversity is for types of learning resources that are not hosted by other Wikimedia projects. Wikiversity is exploring ways to use wiki technology to support learning communities.
- "...the idea here is to also host learning communities, so people who are actually trying to learn, actually have a place to come and interact and help each other figure out how to learn things. We're also going to be hosting and fostering research into how these kinds of things can be used more effectively." (source)
How can Wikiversity and Wikibooks complement each other? 
Wikiversity participants can learn about a topic and then use what they have learned to improve textbooks at Wikibooks or encyclopedia articles at Wikipedia. Helping make textbooks and encyclopedia articles are just two types of learning activities. Many other types of learning activities are being explored at Wikiversity. Take a look at Category:Learning activities.
Many Wikiversity schools, divisions, departments and learning resources have a "Wikibooks" section that links directly to relevant subjects at Wikibooks. See Wikiversity and Wikibooks services for more ways to help Wikiversity and Wikibooks complement each other.
Ideas for classroom use 
Several classes at brick-and-mortar University have utilized Wikibooks for a "real world" class. For example Ethnomedicine was created in a vigorous and rather astonishing fashion. It facilitated both the learning of the individuals of the class, and also made valuable information available for others to use.
You could create a syllabus for your class here at Wikiversity, and then have students create a book as a study or homework tool over at Wikibooks. All of this content would be available for the free use by others at a later date. You can use the protected course template to ensure pages for your students remain consistent with your intentions for the duration of the course.
Wikiversity is still rather new, and you are encouraged to be creative in the ways that you utilize Wikibooks and Wikiversity for your learning goals.
Organization of Wikibooks 
Textbooks at Wikibooks are broken into 8 major subjects, which are then further subdivided into more specific subjects in a hierarchical manner. Each subject page corresponds to a different subject area such as mathematics, computer science, or history. Books on Wikibooks are also organized into alphabetical, Dewey-Decimal, and Library of Congress classification categories, which may be browsed independently from the subjects.
Books for children from birth until age 12 are located in Wikijunior, a sub-project of Wikibooks. Wikijunior books encompass material from all subjects, and are specifically written for children. Some Wikijunior books are intended to accompany a classroom learning atmosphere, but many books are useful for at-home learning between parent and child.