Kevin Robert Brooks noticed that the Asian characters at the foot of the 'Exorcist' letter could be rearranged to spell the words, 'To Kill'. These are obviously the first two words of this novel, but the 'Exorcist' letter also contained an extract from The Mikado about a bird, taken from Act II On a tree by a river- a little tom-tit in this case.
The alphabetical letters LAV letters are present together in the horizontal word SLAVES on the back of the card and underlined in the name Paul Averly on the envelope, but note the three letters at the intersection of the crossed PARADICE and SLAVES running vertically - they spell RAD,
The incorrect spelling of Paul Avery to Averly points towards LEY, to complete the character BOO RADLEY.
In the novel, a local resident by the name of Stephanie Crawford reported she was woken up one night and saw Boo Radley "looking straight through the window at her..... said his head was like a skull lookin' at her". On the tree around the knothole there are the words PEEK A BOO YOU ARE DOOMED.
The musical score to the original motion picture was composed by Elmer Bernstein. Three of the soundtracks are Creepy Caper/Peek-a-Boo, Boo Who and Tree Treasure. In the book, Tree Treasure was a hiding place in the knothole of a tree, where several items were found including a broken pocket watch, an aluminium knife and a ball of twine. These were placed there by Boo Radley for two local kids, Scout and Jem, to discover. The knothole in the tree represented Boo Radley's outlet to the world, and soon the children were to realize that Boo Radley was far from the monster he had been depicted. All the items he placed at intervals in the knothole came in pairs, except the twine - there were two packets of chewing gum, two polished coins, two miniatures of children, and the most interesting of all, was a knife and pocket watch, both on the same chain. Presumably two items, so Jem and Scout could have one each. The 'Eureka' card or 'American Greetings' card, mailed by the Zodiac Killer in 1990, was very similar to the 'Halloween' card in its design, both beginning with the phrase "From your secret pal". The 'Eureka' card included a photocopy, of not one, but a pair of keys on a chain. But who was the "secret pal" - the Zodiac Killer or Boo Radley?