Option 1 -> "few" is the positive form, not comparative, so it doesn't work with "than".
Option 2 -> "less" is comparative but used for uncountable nouns (less water, less time), not countable ones like "anecdotes".
Option 3 -> "fewer" is the comparative form used specifically for countable nouns, which "anecdotes" is.
Option 4 -> "least" is the superlative form, not comparative, so it doesn't fit with "than".
Hence, Option 3: fewer -> Since "anecdotes" is a countable noun and the sentence uses "than" to make a comparison between two books, we need the comparative form for countable nouns, which is "fewer" -> correct