A reference or study book used as a supplement to textbooks, containing summaries, practice problems, and explanations for study or exam preparation.
入試のために何冊か参考書を買って勉強している。
I'm studying by buying several reference books for the entrance exam.
分からないところは参考書の例題を何回も解いて確認した。
I checked the parts I didn't understand by solving the example problems in the reference book several times.
図表が多い参考書は視覚的に覚えやすい。
Reference books with many charts are easier to remember visually.