data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAAA00lEQVQ4y31TMQ4CMQzzg9iZ7wFILMxIt/ABZiZ2FsSGeMKtzHysyJVcGRMYfGmTqImdHJ7XfVsuu0b7us9f+Oe/HacGHvQIQefjvBmPZlx35hCoEmWVJOQjvQN+TvO6g2fdD9tVh2I6u58W4sgKyTN9ToUx3j8oKCDreiRV5cP5eQUJWgnpuiC5pgYZzxz4nL1KNfvUo3fgo8t2c4RupREqdavZZ3x04ByrPfCY74QAzTu1UIVqLzwXv3Y82801F+CJ+R/4HlT6DA2Sm+vhmlT78QZ+7nLGaC/6FAAAAABJRU5ErkJggg==
minecraft-assets/data/1.15.2/texture_content.json
Line 576 in 8cc6915
It is currently:
Which looks like an oak_plank texture?
The block source PNG looks correct:
https://github.com/PrismarineJS/minecraft-assets/blob/8cc6915729564621b5fef0a31d8e60f85d845e81/data/1.15.2/blocks/bookshelf.png