Each repeated title appears in the smiley face with a single letter changed. These altered letters give the message "EASY THEME I CONSTANTLY USE, PLUS E IN MUTANT". The key ideas in this riddle are "easy" and "constant", indicating EASY AS PI, another chestnut. Searching Mutant Sudoku will turn up only one title with +E, namely EASY AS PI+E.
Alternatively, as is typical in word searches, the unused letters provide a second pointer. "I HOPE THE NEW LETTERS GIVE YOU THE IDENTITY OF A THREE WORD EIGHT LETTER CHESTNUT. THE TITLE CAN BE SEEN IN TOMTOM PUZZLES AND BATTLESHIP SUDOKU WHICH ONLY HAS EIGHT NAMES." Knowing the title was used in TomTom Puzzles is more helpful for the next part, but knowing it is in Battleship Sudoku makes it very easy to search there too. Easy as Pi is the first named puzzle in the book, a 6x6 with a sub filling the role of the period in 3.1416.
easyaspi.html is the next page.