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
Q
c
The next (and last) letter is C. Just as a countdown counts backwards, this series is "countdown" spelled backwards.
c is next
y
Oh, dude, let me get my crystal ball out for this one. Hmm, let's see, based on my expert deduction skills, I'd say the next letter is probably... oh, who cares? Like, it's just a random series of letters, not like we're cracking the Da Vinci Code here.
S
T
A new Olympian series, where all the old campers are adults. Percy is not the main character, a boy named Jason is.