Begin with two or three positive aspects of the book. Comment on the author's depth of study, her rich details and descriptions, or his plain writing style. After a small ego boost, everyone is more open to criticism. Tell the writer what you like and don't like about the book. Ask questions to get him to explain ideas or processes that aren't clear to you. Also mention any problems you found with your library or archives. For example, if one of the books you used as a source was difficult to find in a library, let the author know it was hard to read without being able to see the text inside. Finally, tell the author when/if his book will be available again. Remind him that many libraries require books to be checked out for only one user at a time, so let him know if he wants others to have access to the book. Criticism is subjective; there are no right or wrong answers. However, it's important for authors to hear what people have to say about their work. This helps them improve future projects and allows them to connect with other writers.