Exploring Bioinformatics: A Project-Based Approach by Caroline St. Clair and Jonathan Visick is the first bioinformatics book I studied.
At the time, I was very new to the field of bioinformatics (being proficient at some computer programming and knowledgeable in biology) and found the book to be relatively easy to understand. The book is aimed primarily towards undergraduate students at an introductory level of bioinformatics but due to its simplicity, I would recommend it for self-taught undergraduates, autodidacts, or high schoolers who are interested in pursing studies in bioinformatics.
The book is divided into chapters of main topics and techniques necessary in computational genetics and bioinformatics. Each chapter has do-it-yourself type problems utilizing the basic bioinformatics algorithms and programs which are explained within the chapter. At the end of the book is a glossary of important bioinformatics jargon that all bioinformatics enthusiasts must familiarize themselves with.
One of the downsides of this book is that it is written in Perl, albeit Perl being one of the main languages of bioinformatics. If the reader is not familiar with Perl or another computing language, there is an index in the back with the basic computing commands as well as a tutorial near the end of the book on basic Perl programming.
Currently, I have not been successful in locating a free PDF version of the book, so if you are interested in buying it, I would recommend a used copy from Amazon. However, due to its cost, I would recommend bioinformatics enthusiasts to use the Rosalind website (see more here).
biology.jbpub.com/bioinformatics/ is the book’s official website with instructor/student resources and solutions.