There is a huge variety of really good books that you should read to prepare for any biology olympiad or science contest. No matter which books you choose, they all contain the same information which is usually put in other words (or sometimes even the same). Also, the pictures might be different, but apart from that everything is the same. So below you will find recommended books classified according the topics that I found useful while preparing for the International Biology Olympiad.
Biology is nothing without chemistry so you need to know the basics of chemistry as well. My favourite book is the Color Atlas of Biochemistry by Jan Koolman, K. Rohm. It is a pocket-size concise book (approximately 400 pages). What I like most is that every page on the left contains theoretical part and every page on the right depicts diagrams, illustrations, graphs and so on. So, if you remember better by looking at illustrations, I highly recommend to read this little textbook.
Another very useful book is Biochemistry (Lippincott Illustrated Reviews Series) by R. Harvey. It is designed for medical students and therefore is very concise but informative.
Many past participants recommend the Lehninger Principles of Biochemistry, however, I used this book in my 2nd and 3rd year of university and I find this book too detailed for olympiads.
Genetics: Analysis and Principles (WCB Cell & Molecular Biology) by Brooker presents an experimental approach to understanding genetics and what I like most is that there are plenty of problems with explanations and answers. What is more, various laboratory methods are depicted in diagrams so you do not need to work in a lab to get essential practical skills (of course, it is better to do some practice in lab so that not to fail in IBO practical part). Here you can get a better grasp of what is in this textbook.
Another good textbook for genetics is Genetics: From Genes to Genomes, 5th edition by Hartwell.
It is not a secret that the Bible of Biology is Campbell Biology (10th Edition). It is a good book and it covers all fundamental biology topics, nevertheless, some topics are discussed only concisely so some good books in addition to Campbell’s could come in handy.
Life: The Science of Biology by David Sadava et al. is a book similar to Campbell’s but contains some different information as well as more comprehensive images. There are quite good study resources from this book here and here.
I personally adore Biology by Raven et al. I found here some facts I did not read in any other book, for example, Raven interestingly presented motives of proteins or unique examples of behaviour ecology. Here you can find some nice chapter examples.
For human body anatomy and physiology great books are Human Physiology: An Integrated Approach (7th Edition) by Dee Unglaub Silverthorn or Vander’s Human Physiology. The newly revised Human Physiology: An Integrated Approach Fourth Edition strengthens the coverage of the “big picture” themes in the study of physiology and helps tie concepts together in a logical framework for learning. Vander’s Human physiology reflects some of the major themes of the textbook: homeostasis, exercise, pathophysiology, and cellular and molecular mechanisms of body function. You can take a look at one of the chapters of this book here.
I also read Human Physiology: From Cells to Systems by Sherwood. This book is meant for the one-semester Human Physiology course typically taken by allied health and biology students. The start chapters introduce basic chemical and biological ideas to offer students with the framework they should comprehend physiological principles. Moreover, the chapters promote conceptual understanding.
As far as practical part of IBO is concerned, I have not found a better book than Practical Skills in Biomolecular Science, 5th ed. by Rob Reed. This textbook underpins any practical course in biochemistry, biomedical science, genetics, immunology and microbiology. Not only it includes numerous exercises with answers, but also foster study and examination skills, laboratory and analytical skills, investigative techniques and analysis of experimental data. Some teachers also recommend Biochemistry, 4th Edition by Donald Voet, Judith G. Voet but this one is as comprehensive as Lehninger’s.
Modern Biology: Skills Practice Labs: Includes Dissection Labs is a nice book to develop practical skills as well.
For microbiology, I would suggest Brock Biology of Microorganisms (14th Edition). This book for biology, microbiology, and other science majors balances cutting edge research with the concepts essential for understanding the field of microbiology.
I would like to suggest the best book for molecular biology, Molecular Biology of the Cell by Bruce Alberts, et al. This is book is a big one, a hard one, an interesting one, a useful one. From my point of view, current and upcoming IBOs are focusing on molecular and cell biology because these fields are developing so rapidly and thus these branches of biology are perfect source for olympiad problems. So try to read it and understand it.
If you want something cheaper than Alberts but equally useful, try Molecular Biology of the Cell, Fifth Edition: The Problems Book. The Problems Book helps students appreciate the ways in which experiments and simple calculations can lead to an understanding of how cells work by introducing the experimental foundation of cell and molecular biology. Each chapter will review key terms, test for understanding basic concepts, and pose research-based problems. The Problems Book has been designed to correspond with the first twenty chapters of Molecular Biology of the Cell, Fifth Edition.
Animal Physiology presents all the branches of modern animal physiology with a strong emphasis on integration of physiological knowledge, ecology, and evolutionary biology.
Biology Of Animals presents coverage of animal form, function, diversity and evolution. In this sixth edition, the cladistic perspective (phylogenetics systematics) has been added to the discussion of animal phyla, and cladograms of the relationships are included.
My personal favourite is Integrated Principles of Zoology by Hickman.