Watchtower 5/18/1957, pp.313-315 Appreciating Basic Christian Publications
https://wol.jw.org/en/wol/d/r1/lp-e/1957367

Their zeal for knowledge may even prompt them to dig back into things that were published long before they came into the truth, expanding and deepening their understanding, and ever growing in Christian maturity.
https://www.jw.org/finder?wtlocale=E&docid=1957367&srctype=wol&srcid=share&par=10
 
Do you dig out older publications to expand and deepen your knowledge on subjects about which questions arise? Have you really studied these earlier publications?
https://www.jw.org/finder?wtlocale=E&docid=1957367&srctype=wol&srcid=share&par=11
 
How is your background of knowledge? ... Never think: "Oh, I know most of that." For you will find, indeed, that you do not and that you will be strengthened by your additional study.
https://www.jw.org/finder?wtlocale=E&docid=1957367&srctype=wol&srcid=share&par=12
 
And once you have completed your study of them you can then look to even older publications... Only by studying earlier publications and digging back through previous issues of The Watchtower kept in the library at your local Kingdom Hall. There is much in the way of spiritual riches and aids toward mature knowledge in these earlier publications, and their study is most certainly worth your time.
https://www.jw.org/finder?wtlocale=E&docid=1957367&srctype=wol&srcid=share&par=16

Kingdom Ministry 1/7/1988 Page 7
"You will find valuable information in the older publications."
https://wol.jw.org/en/wol/d/r1/lp-e/201988007#h=7

The Watchtower 12/15/1990, Page 26
“For over a century, delightful, correct words of truth covering every aspect of life have been presented in the Watch Tower Society’s publications and widely distributed in many languages. Many families and individuals have thus been able to accumulate in their own homes a library of reliable reference works that focus on the Bible. In addition, most congregations of Jehovah’s Witnesses have fine libraries for use at their Kingdom Halls.”
https://wol.jw.org/en/wol/d/r1/lp-e/1990928#h=3