Does a book require a rabbinic approbation? R. Moshe Sofer, the famous Hasam Sofer (Responsa, Hoshen Mishpat no. 41) writes that the established custom is for author to publish a rabbinic approbation on every book, even on very old ones.
R. Hershel Schachter discusses this issue in his approbation to the first volume of R. Baruch Simon’s Imrei Barukh (of which volume 2 has been recently published but which I have not yet obtained). After quoting the Hasam Sofer, he refers to the laws of demai. Demai is produce from which we are uncertain whether the required priestly and levitical portions have been removed. Food obtained from a haver is free of such uncertainty. In order to become a haver, which has implications to the laws of purity as well, one has to publicly and officially accept strict compliance to those laws. However, R. Schachter states, presumably referring to Bekhoros 30b, someone appointed to a respected position of teaching Torah need not go through such an official procedure. One can assume that he is trustworthy on such matters (cf. the glosses of R. Ya’akov Emden, ad loc.). Therefore, suggests R. Schachter, the same can be said of approbations. Someone who teaches and spreads Torah on a widespread basis can be assumed to be trustworthy and does not need an approbation.