noun
primer; introductory book; beginner's guide
A book designed for newcomers to a subject, providing basic knowledge and step-by-step instruction.
プログラミングの入門書を買った。
I bought a beginner's guide to programming.
この入門書は初心者にもわかりやすい。
This introductory book is easy to understand even for beginners.
手引書 is a handbook or manual that provides practical guidance, whereas 入門書 focuses on introducing the fundamentals of a subject.
Compound of 入門 (にゅうもん, 'entering a gate', i.e., introduction) and 書 (しょ, 'book'). The meaning is transparent from its components.