I is the next letter.
L
E
Next three letters are: e n d
Between A and D are two letters, between D and E are no letters, between E and H are two letters, and between H and I are no letters. Therefore, there would be two letters between I and the next letter, so it would be L.
E. H. D. Sewell died in 1947.
E. H. D. Sewell was born in 1872.
The next letter is D. The pattern is every fourth letter. Ie. a b c D e f g H i j k L m n o P and so on...
D E H. Frear has written: 'Pesticide handbook - Entoma'
The obvious one is "L" as you start with A then add 3 letters = D, then add 1 letter = E, then add 3 letters = H, then add 1 letter = I, then add 3 letters = L.
D
a b c e d h so r h e u i d d h w i d x u f sif s