I think I’ve made some progress! Most likely “the first of each” had to refer to something in the book, but what? Well, it’s a message, so probably the first word or letter, and “of each” would probably refer to word, line, sentence or page. (I noticed some of you had the same idea.)
I tried taking the first word of each line, then each sentence, and at last each page, but the result was complete nonsense in all cases. Yes, the riddle said cryptic, but not nonsensical. It couldn’t be a cipher either, as those are character based and not word based.
It has to be each letter. But of what? Well, if it was each word, it would be a very long hidden message. Also, when it comes to steganography, the ratio between the obvious and the hidden data matters. It’s hard to hide something if what you’re hiding constitutes a very large part of the whole. If nothing else, it would be very hard to construct meaningful sentences which would not arouse suspicion if the first letter of each word, or even each line or sentence, had to be something specific.
The first letter of each page must be it. It would certainly be practical enough to work into a book without being noticeable.
This is the cryptic message:
HWMG RJKB GEDR OBOO NHNN TLRQ YQDN KXJQ BOIQ ONNU EGAL ZCZW UEGE IHII FITZ KCKD IXHM LAXR LWKI JZEB GIEW DMXA UYWV BEUW DMBR RIPB NUEV AMMH VLCP SSPH PEHT YVKU TCYP WOHC TRJK DNKU WBDD ZQKV DPCD YMRO DHPP HNXZ WAAO LQZB NGFZ NWMG JRKV AUIV PANF NLHL LAXL YIEF CLYB DDKK AKHT PDJL DVVU UXNN PZWA EOIF ALSO VYWK TAKH IIFO GRHQ RWRV ZMBL RUAL TNJD BMEO ISKV HEJI KZGU EGNM CSFI UMZR JDJL LOJW NMBO IGOW EIKG AASR FBAL VHVQ UWTR VDZB GIJL PEHL CEAB HMVS WODT NHJB XFFX NNHV VGKE MTNR NQFH K
As I suspected, cryptic message as in encrypted. Though it’s not a simple substitution cipher, I already tried that. I’m guessing it’s a Vineighre cipher, those were very popular in that era (and were generally thought to be unbreakable at the time). It would also make sense with the last line of the riddle, “only if you are the key”, as Vineighre ciphers use decryption keys.
Riddles, hidden and encrypted messages, this is really exciting! Optimally, this will lead to some amazing discoveries, but if nothing else, at least I will have had fun finding solutions. I just hope my assumptions are correct, or I will be trying to crack a “cipher” that’s just a string of arbitrary nonsense, and that will be rather futile.