What I meant was their job is to author such books. Not that they should be paid separately for them. Depending on the country academicians might be underpaid, but that's an issue on its own.
What I meant was their job is to author such books
Except in many cases it isn't. Most of the people I know who have written academic books did so off the clock and on their own time. Sure the university let them use their university office and resources, and obviously much of the research the book is based on is research they'd already done as part of their job, but the actual writing time and any additional research they had to finance from sources unrelated to their university job.
While that does occasionally happen, it's definitely not true in general. The people I know who wrote academic books did so by taking a sabbatical from their university job and/or working evenings and weekends for the actual writing. Occasionally they can apply for a separate writing grant to cover their lost salary, but that's completely separate from their day job.