Resources (OER) are becoming more prevalent as scientists and educators attempt to make information more accessible. Studies have shown that OERs, which users have full access to without fees, do not diminish learning, and for some people improve learning outcomes (reviewed in Mullens & Hoffman, 2023). In fact, my university, which has 79% of students on federally-funded Pell Grants, 1 now requires faculty to use OERs (specifically no-cost or low-cost textbooks) whenever possible. Students in the U.S. can spend upwards of $1,000 per year for textbooks; reducing that cost can increase accessibility and improve outcomes. Thus, I was excited to see that there was a new open-access marine mammal textbook released in 2023 that would finally allow me to join the bandwagon of open access textbooks for my students. Historically, we have struggled to find a textbook to use (see Read, 2012, for a historical review of marine mammal textbooks). I had been using Hoelzel's Marine Mammal Biology: An Evolutionary Approach for 15 years in my classes. It covers most of the subjects of biology of marine mammals, from evolution and taxonomy to physiology to behavior. However, it was slightly outdated and expensive for my students to use. So, I immediately downloaded Marine Mammals: A Deep Dive into the World of Science, hoping I could assign it the next time I teach Marine Mammalogy to undergraduates.I reviewed the first chapter with my students (graduate and undergraduate) and asked them to help me evaluate it as to who can use it, what can it be used for, and what are its limitations. After that exercise, I found the same conclusions to be true for all the chapters. Overall, it is a good start for students learning about marine mammals, especially in the North and Baltic Seas. However, because it only covers North Sea marine mammals, i.e., harbor porpoises and three seals (harbor, gray, and ringed seals), it is limited in its scope for other audiences. The preface indicates that it is geared towards high school students; at times it seems to be, but at others, it is too high level for even undergraduates. There are clear learning goals at the beginning of each chapter that are good guidelines for teachers and students alike. However, some chapters never really strike a balance of good information without overwhelming the reader with new terms. There are some brilliant moments in the book, especially in the Current Topics in Research boxes that highlight specific studies relevant to the information in the main body of the chapter. Also, the activities at the end of each chapter are well planned out and would be useful in a high school or college classroom to emphasize lessons learned from the chapters. Most of the chapters are on threats to marine mammals, including acoustics, chemical pollutants, plastics, and fisheries, which is helpful when discussing conservation of marine mammals.