CONTRIBUTIONS TO THE SOCIOLOGY OF LANGUAGE brings to students, researchers and practitioners in all of the social and language-related sciences carefully selected book-length publications dealing with sociolinguistic theory, methods, findings and applications. It approaches the study of language in society in its broadest sense, as a truly international and interdisciplinary field in which various approaches, theoretical and empirical, supplement and complement each other. The series invites the attention of linguists, language teachers of all interests, sociologists, political scientists, anthropologists, historians etc. to the development of the sociology of language.
Definitely the best single-book introduction to the early history of Esperanto (including Esperanto language works). Forster writes very concisely, and particularly for the years 1887-1920ish he is very good at covering the terrain without being either too sparse in the detail or getting bogged down in it. Despite the book being written from the late 60s and published in 1981, it has a timeless feel to it. You don't get the impression that his conclusions are out of date, or his approach is dated, or that a more modern work could cover the topic better than he did.