This question assumes that the learner of a second language (hereafter abbreviated as L2L):
has at least an undergraduate Major in linguistics.
is learning, from scratch, a common language such as one these 100.
Popular books or resources on language learning may prove shallow and superficial, because e.g.:
- they fail to explain exceptions to rules, or help the L2L whom memorisation has failed for understanding nuances (e.g. as a method to acquire adpositions).
- knowing phonetics and phonology, the L2L may prefer precise IPA transcriptions over 'non-phonemic' or 'newspaper' systems of pronunciation respelling.
- knowing historical linguistics, the L2L may prefer an etymological approach to new words.