For a Digambara-based, authoritative study of gochara and the rules of alms-food, the most important primary book is:

1) _Mūlācāra_ This is the most direct and complete Digambara source for the conduct of monks, including gochara, āhāra-vidhi, what is acceptable, what is prohibited, and the discipline around receiving food.

If you want a single book to understand the subject in depth, start with _Mūlācāra_.

Other important Digambara texts

2) _Bhagavatī Ārādhanā_ Very important for monk conduct and careful observance of food discipline.

3) _Ratnakaraṇḍa Śrāvakācāra_ Helpful for a layperson. It explains what a householder should avoid, especially in relation to food, donations, and proper Jain conduct.

4) _Puruṣārthasiddhyupāya_ Good for understanding ahiṃsā in food, prohibited items, and ethical discipline in a simple way.

Best practical recommendation

If your aim is:

• complete Digambara rules of gochara → _Mūlācāra_
• simple understanding for a commoner/श्रावक → _Ratnakaraṇḍa Śrāvakācāra_
• deep traditional discipline → _Bhagavatī Ārādhanā_

In short

Exact main book: _Mūlācāra_ Best supporting book for a commoner: _Ratnakaraṇḍa Śrāvakācāra_

If you want, I can also give you a Digambara list of prohibited foods and improper food methods in simple points.