Vanilla Connected Textures
This resource pack adds connected textures effect for various materials, and it works without OptiFine or any other mods! Compatible blocks include metals, gems, polished stones, and sandstones.
Implementation features: Since it uses vanilla methods, blocks connect to any other blocks (except for leaves, glass, iron bars, and other similar blocks). Despite this, the result looks quite natural and beautiful.
Performance: The pack has been optimized to the maximum, so you won't experience FPS drops or performance degradation.
Compatibility with other resource packs: Currently, compatibility is limited to bookshelves and sandstone blocks, as connecting other textures looked unsatisfactory.

Technical details: The effect is achieved through reverse face culling rendering, which makes blocks appear connected to each other.
The bookshelves look especially impressive in stronghold libraries - just the view is worth checking out!