Part of a conductor’s training is learning how to read a score — the ups, downs, and arounds of the musical dynamics, expressions, staves, and clefs.
You must know your C clefs from your G clef, your F clef from your baritone. (Just kidding! The last two are the same.) And, of course, you must know which instruments are written in transposition, and by how much.
Here, then, are the best books for pianists looking to practice reading full scores:
Preparatory Exercises in Score Reading, Morris & Ferguson
Music for Score Reading, Melcher & Warch
Bach Chorales 1-91 in Open Score, ed. Boyd & Riemenschneider
Partiturspiel (Vol. 1-4), Creuzburg (Published by Schott; these are excellent, but tricky to find in the US)
Score Reading Exercises (Vol. 1 & 2), Lang
Score Reading of Orchestral Music: Guide for Conductors and Music Readers, Metelska-Räsänen (This is the academic, one-stop-shop textbook — you can skip this if you just want materials for reading practice)