I think those who are not in the habit of doing so lose the pleasure of reading when the spare evening or the long railway journey or the rainy day comes.
スラッシュは、接続詞の前、主語を修飾する節の終わりに入れます。
I think /those who are not in the habit of doing so/ lose the pleasure of reading/ when the spare evening or the long railway journey or the rainy day comes.