Even though I could read sheet music, I mostly played piano by ear until around high school when I decided to properly learn to sight-read. At the time, my access to musical resources was limited, so I borrowed the Episcopal Church hymnal from our church.
The great thing about hymnals is that they are choral in nature, usually with four voices, but at the same time, they are rhythmically simple in nature since they are intended to be sung, allowing you to purely concentrate on the notes themselves.
I ran through it sequentially for months and found that my sight-reading capabilities measurably improved.