Definition of "immersed"

ON THE NARROW GAUGE. By P. B. Whitehouse. Nelson 30s. [...] In an introductory note Mr. Whitehouse disarms any criticism from narrow-gauge aficionados as immersed in the subject as himself by pointing out that his book has no pretensions to the status of a new narrow-gauge textbook; [...].

1964 July, “New Books”, in Modern Railways, page 74