This guidebook might come in handy for the trip.

Literal

Trip [on-で] this guidebook [subject-が] useful might-be [emphasis-よ].

役に立つ (to be useful/helpful) is a set expression: 役 (role) + に立つ (to stand). かもしれません (might) softens the suggestion. よ adds a gentle assertive push — 'you know, this might help.'