Biology is one of the fundamental scientific disciplines, and in terms of the impact on our everyday life it is perhaps one of the most important. It is also one of the scientific disciplines with the greatest amount of new research, and hardly a year passes without some new profound discovery being made. Sometimes even the experts are hard to keep up with all of the recent developments, and new and updated textbooks are published on a yearly basis. Unfortunately, the prices of new textbooks have been steadily increasing over the years, so it is incredibly refreshing to come across a well-organized free textbook that can be used in introductory biology classes.
This is by and large a very approachable and well presented introductory biology textbook. It is written with an advanced high school student or beginning college student in mind. Topics covered include foundational topics, chemical basis of life, cell structure and function, photosynthesis, cell division and reproduction, genetics, biotechnology, evolutionary theory, biological classification, ecology, human body, various tissues and organs, and many others.
Each chapter ends with a several questions, most of which are not quantitative. In fact, this is not a very quantitatively oriented textbook, and it stresses critical and scientific thinking over quantitative skills. However, it is a very interesting and well organized textbook and provides an excellent introduction to biology. In addition to questions, there are also numerous vocabulary terms listed at the end of each chapter. These provide the student with an opportunity to systematize the knowledge gained in the preceding chapter. There are also numerous references for further study and links to other outside resources.
The book is replete with many illustrations and photographs, and they are generally of high quality and on the average much better than in most other CK-12 textbooks.
This book is available under the Creative Commons License through the CK-12 foundation, which means it can be reprinted, modified and resold if necessary. It is also a fairly large file, the pdf version being over 2000 pages long, so be prepared for a long download time.
The Kindle formatting of this textbook leaves something to be desired. The book was originally typeset in LaTeX, and this did not translate all that smoothly into the Kindle format. I've found that getting this textbook on other e-readers or computers in the epub format rendered it much more satisfactorily.
This is not the flashiest textbook that you will come across, but in my opinion it gets the job done.