Lorna Doone by R.D. Blackmore (1869) is the unavoidable starting point. The novel is set in the Doone Valley a few miles east of Lynmouth, and the Exmoor landscape — moor, gorge, and river — is as much a character as the human cast. It's a Victorian romance that takes its setting very seriously. Read it before you walk up to Badgworthy Water and you'll find the landscape more layered.
The Flood by Roxy Huxley, and the journalism and documentary record around the 1952 Lynmouth disaster, repay attention for anyone trying to understand the village and why so much of it looks relatively modern. The flood killed 34 people, destroyed 93 houses, and remade the waterfront overnight.
Exmoor: The Landscape and Its People by a range of contributors in the Exmoor Review — the annual journal of the Exmoor Society — publishes good local history and natural history essays. Not beach reading, but genuinely useful for understanding the moor and its communities.