Lie' (to recline) has the past participle 'lain'. 'Lay' (to place) has the past participle 'laid'. The books are reclining; therefore, 'lain' is correct.
Lie' (to recline) has the past participle 'lain'. 'Lay' (to place) has the past participle 'laid'. The books are reclining; therefore, 'lain' is correct.