Stewards of the Flame

According to the author's newsletter, this series was originally supposed to consist of just one sequence. However, based on reader feedback, it was split into two, "Hidden Flame" (volumes 1 and 2) and "Rising Flame" (volumes 3 and 4). The second sub-series can be read without having read the first sub-series although it spoils the latter. Although these books are set in the "Anthropological Service" universe, unlike the "Elana" books, they are aimed at adults and older teenagers as opposed to middle-school readers.