"On" and "in"-well,there is often a wide difference between these two little words.A book on English is one that deals with the English language; a book in English is one that is written in the English language.The former may be written in Han,French,German,or any other language; the latter may deal with any subject under the sun[1].
Books on English are intended for learners of the language[2]; and it is very gratifying to note that far more such books are now available than fifty years ago or more.I myself have written several,and am glad to say that I often receive letters telling of their usefulness to learners of English.
But I must give a word of warning here.Language is not an exect science,nor a mere matter of principles,rules,exceptions,definitions,formulas,and diagrams.Books on English are helpful,but no one can master English by reading such books only.Perhaps the very fact of[3] there being so many such books today accounts for the frequency with which we meet with people who can talk and write about English without being able to talk and write it[4].This is due to their ignorance of the fact that without wide and careful reading the mastery of English is impossible.
I would advise all learners of English to devote more time to reading books in English and less time to reading books on English.What book in English are you reading? Is it a book of stories or essays or travel sketches? Or is it a biography or a diary or merely a textbook of history or geography? Well,if you happen to spend your days and nights on books on English,it is time you acted on my warning[5].
NOTES
[1]under the sun 在天下
[2]the language=the English language
[3]the very fact of 恰恰是……这个事实
[4]account for the frequency with which we meet with people who can talk and write about English without being able to talk and write it.就是我们时常碰到能在谈话和写作中讲述英语但（他们自己却）不能讲英语和写英语的原因。
[5]it is time you acted on my warning 现在你该实行我的警告了（注意“it is time…”中动词用过去式）