noun
specialized book; technical book; treatise
Refers to a book written for experts or advanced learners in a specific field, as opposed to introductory or general-audience books.
この専門書は大学院生向けに書かれている。
This specialized book is written for graduate students.
彼は医学の専門書を何冊も翻訳した。
He has translated many medical treatises.
参考書 is a reference book or study guide, often for students, while 専門書 is a specialized monograph or treatise.
Compound of 専門 (specialty, field of expertise) and 書 (book, document).