"The Poacher's Handbook" by Ian Niall (pub. 1950).
Also "I Walked by Night" as mentioned above. Both good reads.
Also edited by Lilias Rider-Haggard is "The Rabbit Skin Cap". Not about poaching specifically, but quite a bit of poaching in it.
Also "A Poacher's Tale" - the story of Alfred Curtis, as recounted by Fred Speakman.
There's plenty more - that's just a few I can spot easily on the shelf in front of me as I type...