primer; introductory book; beginner's guide
A book designed for newcomers to a subject, providing basic knowledge and step-by-step instruction.
I bought a beginner's guide to programming.
This introductory book is easy to understand even for beginners.
入門書 is aimed at absolute beginners starting a subject from scratch, while 手引書 can also serve as a practical reference for those already familiar with the basics.
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.