Istanbul: Solution

This puzzle is not a standard cryptogram. Instead, each word is a Caesar shift by the number of letters in the word. This yields:

TWAS BRILLIG AND THE SLITHY TOVES DID GYRE AND GIMBLE IN THE 

WABE ALL MIMSY WERE THE BOROGOVES AND THE MOME RATHS OUTGRABE 

BEWARE THE JABBERWOCK MY SON THE JAWS THAT BITE THE CLAWS 

THAT CATCH BEWARE THE JUBJUB BIRD AND SHUN THE FRUMIOUS 

BANDERSNATCH HE TOOK HIS VORPAL SWORD IN HAND LONG TIME THE 

MANXOME FOE HE SOUGHT SO RESTED HE BY THE TUMTUM TREE AND 

STOOD AWHILE IN THOUGHT AND AS IN UFFISH THOUGHT HE STOOD THE 

JABBERWOCK WITH EYES OF FLAME CAME WHIFFLING THROUGH THE 

TANGLE WOOD AND BURBLED AS IT CAME ONE TWO ONE TWO AND 

THROUGH AND THROUGH THE VORPAL BLADE WENT SNICKER SNACK HE 

LEFT IT DEAD AND WITH ITS HEAD HE WENT GALUMPHING BACK AND 

HAST THOU SLAIN THE JABBERWOCK COME TO MY ARMS MY BEAMISH BOY 

O FRABJOUS DAY CALLOOH CALLAY HE CHORTLED IN HIS JOY 

This is almost the complete text of "Jabberwocky", save for the word "tangle", which is actually "tulgey" in the poem. The answer is, therefore, TANGLE.