This book has comprehensives and well-organized overview of the grammar with lots of examples. I found it easier to learn from than lessons, like most textbooks seem to have. And in spite of the fact that the book is 100 years old, Esperanto hasn't changed enough to make any difference. Sure, people are more like to say "esperantio" than "esperantujo".
Along the way, he’s made the acquaintance of old friends and new, as he follows the clues, solves the riddles, and earns the intense rewards the gems provide. Until the trail to the final stone leads to matters of life and death, a meeting with an underworld kingpin, and a possible connection to Persephone’s philandering husband-- as well as the truth about who sent him on that quest, family secrets unimagined, and pleasures undreamt of...