It's the letter D of course! Haven't you heard of Jason D?!
The main character in the next Percy Jackson and the Olympians series is named Jason. There is not much information given on him, or the next Camp Half blood series.
what is the next letter e h l q
X
S
c
Q
The next (and last) letter is C. Just as a countdown counts backwards, this series is "countdown" spelled backwards.
c is next
y
S
T
A new Olympian series, where all the old campers are adults. Percy is not the main character, a boy named Jason is.