The Esperanta antologio: the anthology of Esperanto poetry. Get hold of the first edition, rather than the second; yes, the first edition stops at 1957, but it has commentary, which is very useful, and the 1984 edition inexcusably got rid of it.
Proverbaro Esperanta: Zamenhof’s collection of multilingual proverbs, complete with earworm rhymes in the Esperanto versions. Instant culture for the language, and lamentably not used much in Esperanto (with the exception of the translation of Asterix the Gaul).
Memkritiko [‘Self-Criticism’] by Victor Sadler: the best poems in the language. Yes, it’s a big call. Very postmodern (especially for 1967) with the “editor” putting down all the poems, in a tense dialogue; exquisite miniatures, that have passed unremarked because people are looking for “greatness” instead of quality. This 2014 review doesn’t quite get it, but it does indicate that there are plenty of copies still available for sale at €2.70.