This particular edition of The Secret of the Old Clock is a reprint of the original, 1930 text and is NOT THE 1959 REVISION. An easy way to tell is by the cover art. If Nancy Drew is walking through the woods while holding a clock, it is the 1930 text. If she is kneeling down, it is the rewrite.
With this in mind, it is important to understand that there is an extensive section involving racial stereotypes concerning an African American character that some may find offensive. For this reason, this book, in my opinion, is not really suitable for younger children. However, adults and older children who are familar with the Nancy Drew series and have read the 1959 version of this book will certainly enjoy reading this original text.
Having read both the 1930 and the 1959 version, my personal opinion is that the 1930 version is the better book. Unlike in the rewrite, we gain more insight into why Nancy and the Topham sisters do not get along, for example. There is also better character development and these characters seem more authentic. The rewrite suffers somewhat in these respects and should have simply been rewritten to remove the offensive material while leaving the remaining text mostly intact.