The first book was written in 1942 - it's mentioned explicitly in LADR. It doesn't mean the concepts didn't exist - they did, Frobenius even built a brilliant theory around them (representation theory), but the subject was defined quite loosely - apparently no one cared to collect the results in one place. It doesn't even matter much: I remember taking the course in 1974, and it was totally different from what is being taught today.