。
The post office is next to the library.
Literal
Post-office [topic-は] library-of next [at-に] exist.
隣 ('next to, neighbor') is the spatial noun for immediate adjacency. Takes a genitive の-link: 図書館の隣 ('next to the library'). Distinct from 横 ('side, beside'), which is more about the lateral side than immediate neighbor, and from 近く ('near'), which is broader vicinity.