Hargrave Jennings (?) - Cultus Arborum: A Descriptive Account of Phallic Tree Worship (1890)
Archive.org direct link
Open Library main page
Phallic tree worship? Sure, why not - except this book is mainly a compendium of tree worship and little else. Even a contemporary notice in the Theosophical Review griped as much ("we fear however that literary incubi will find too much about Tree Worship and too little about Phallicism to delay them long over the present exposition") before it then veers off on its own tangent.
The book was privately published with no author indication but some modern sources claim it was the work of occultist Hargrave Jennings (1817-1890). This doesn't seem to be a particularly firm attribution but I'll leave it.