Did Loudon think of writing a book on landscape architecture?

Probably not. But he did plan to work on a book with Gilbert Laing Meason which is very likely to have used the term 'landscape architecture' in the title and, since Loudon had no expertise in painting, Loudon's contribution is most likely to have been about the relationship between buildings and sites.

See also: Gardenvisit.com appreciation of John Claudius Loudon.