noun which may take the genitive case particle 'no', noun, used as a suffix
indispensable; essential; must-have
Used as a suffix or with の to describe something one should always carry or have at hand, such as a reference book, tool, or item.
この辞書は学習者必携の一冊だ。
This dictionary is an indispensable book for learners.
旅行必携のアイテムをリストにまとめた。
I listed the must-have items for the trip.
noun
Usually appears in book titles to mean a compact reference work meant to be kept close at hand.
『山歩き必携』という本を買った。
I bought a book called 'A Handbook for Mountain Walking'.
このシリーズは現場必携として重宝されている。
This series is valued as an on-site manual.