Unquote #9 answer

The quote

Three weeks is an eternity to a brain like mine. Without the constant stimulation, my little grey cells will starve and die.

The puzzle as it was set

AXZPP SPPON UN RY PAPZYUAQ AV R JZRUY IUOP BUYP. SUAXVEA AXP HVYNARYA NAUBEIRAUVY, BQ IUAAIP KZPQ HPIIN SUII NARZGP RYL LUP.

The key

Each code letter stood for one real letter (code letter, then the real letter it hid):

R → AJ → BH → CL → DP → EK → GX → HU → IO → KI → LB → MY → NV → OZ → RN → SA → TE → UG → VS → WQ → Y

The most frequent code letter was A (14 times), which stood for T. Short words, apostrophes and repeated letters are the usual way in: a one-letter word is almost always A or I, and THE is the commonest three-letter word in English.