There wasn't enough light, so I couldn't read the book properly.
Literal
Light [subject-が] sufficient [not-なかった] [because-ので], book [subject-が] well could-not-read.
十分に ('sufficiently') + ない = 'not sufficiently, not enough.' The potential form 読める ('can read') takes が for its target (honoring the object-to-subject shift of potential verbs). A well-constructed cause-and-effect sentence.