意見 and 感想: views and impressions
Distinguish views offered in a discussion from impressions of a work or experience.
A view offered for discussion
意見 is read いけん and refers to a judgment or proposal about an issue. 「計画について意見を聞かせてください」 asks for the listener's views on a plan.
「私は別の意見です」 says that the speaker holds a different view. It does not automatically mean that they reject the other person's experience or feelings.
An impression from an experience
感想 is read かんそう and refers to thoughts or feelings after seeing, hearing, or doing something. 「映画の感想を話しました」 means that the speaker shared their impressions of a movie.
「初めて参加した感想はどうですか」 asks for impressions of participating for the first time. It can invite a broad account of the experience, not just proposals for improving the event.
Moving from an impression to a proposal
「説明が長いと感じました」 gives an impression of the experience, while 「説明を短くしたほうがいいと思います」 proposes an improvement. Both can arise from the same experience.
感想 can include evaluation and thought, so do not divide the words into emotion alone versus reason alone. Match your response to whether the listener wants an account of the experience or a judgment or proposal about the issue.
What is a piece of writing about your impressions after reading a book usually called?
Show the answer and why
It is called 「読書感想文」. It describes thoughts and feelings about the work and may also include the writer's 意見.