True, there is an occasional mention in his column of a point raised by a correspondent, and the Letters section of the Magazine prints a comment from time to time, but an important feature of the books is their inclusion of far more writer-reader interaction than one might suspect from reading the column alone.
( ( ( ( ( True ( , ( ( ( ( there ( is ( ( an ( occasional mention ) ) ( in ( ( his column ) ( of ( ( a point ) ( raised ( by ( a correspondent ) ) ) ) ) ) ) ) ) ) , ) and ) ( ( ( the ( Letters section ) ) ( of ( the Magazine ) ) ) ( ( prints ( ( a comment ) ( from time ) ) ) ( to time ) ) ) ) ) ) , ) but ) ( ( ( an ( important feature ) ) ( of ( the books ) ) ) ( ( is ( ( their inclusion ) ( of ( ( far ( more writer-reader ) ) interaction ) ) ) ) ( than ( one ( might ( suspect ( from ( ( reading ( the column ) ) alone ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (INTJ (UH True)) (, ,) (S (NP (EX there)) (VP (VBZ is) (NP (NP (DT an) (JJ occasional) (NN mention)) (PP (IN in) (NP (NP (PRP$ his) (NN column)) (PP (IN of) (NP (NP (DT a) (NN point)) (VP (VBN raised) (PP (IN by) (NP (DT a) (NN correspondent))))))))))) (, ,) (CC and) (S (NP (NP (DT the) (NNP Letters) (NN section)) (PP (IN of) (NP (DT the) (NNP Magazine)))) (VP (VBZ prints) (NP (NP (DT a) (NN comment)) (PP (IN from) (NP (NN time)))) (PP (TO to) (NP (NN time)))))) (, ,) (CC but) (S (NP (NP (DT an) (JJ important) (NN feature)) (PP (IN of) (NP (DT the) (NNS books)))) (VP (VBZ is) (NP (NP (PRP$ their) (NN inclusion)) (PP (IN of) (NP (ADJP (RB far) (RBR more) (JJ writer-reader)) (NN interaction)))) (SBAR (IN than) (S (NP (PRP one)) (VP (MD might) (VP (VB suspect) (PP (IN from) (S (VP (VBG reading) (NP (DT the) (NN column)) (ADVP (RB alone))))))))))) (. .)))
There is much more writer-reader interaction present in the books then in the columns. 
( There ( ( ( ( ( is ( much more ) ) ( ( writer-reader ( interaction present ) ) ( in ( the books ) ) ) ) then ) ( in ( the columns ) ) ) . ) )
(ROOT (S (NP (EX There)) (VP (VBZ is) (ADVP (RB much) (RBR more)) (NP (NP (JJ writer-reader) (NN interaction) (NN present)) (PP (IN in) (NP (DT the) (NNS books)))) (ADVP (RB then)) (PP (IN in) (NP (DT the) (NNS columns)))) (. .)))