I got a copy of this book during the weekend: a manual of Latin style, written in Latin. The first few chapters include definitions of style and lists of archaic words and neologisms, with those that ought to be used and the ones better avoided. For instance, the ancient words cascus (i.e. priscus, vetus) and cluere (dici, appellari) are omnino obsoleta, however actutum (cito, illico, confestim) and adagium (proverbium) possunt interdum usurpari.