In this study, I investigate the effect of auditor-provided tax services (ATS) on firms’ levels of book-tax differences and investors’ mispricing of book-tax differences. The joint provision of audit and tax services has been a controversial issue among regulators and academic researchers. Evidence on whether ATS improve or impair the overall accounting quality is inconclusive as a result of the specific testing circumstances involved in different studies. Book-tax differences capture managers’ earnings management and/or tax avoidance intended to maximize reported financial income and to minimize tax expense. Therefore, my first research question investigates whether ATS improve or impair audit quality by examining the relation between ATS and firms’ levels of book-tax differences. My results show that ATS are negatively related to book-tax differences, suggesting that ATS improve the overall audit quality and reduce aggressive financial and/or tax reporting. My second research question examines whether the improved earnings quality for firms acquiring ATS leads to reduced mispricing of book-tax differences among investors. Recent studies document that despite the rich information about firms’ future earnings contained in book-tax differences, investors process such information inefficiently, leading to systematic pricing errors among firms with large book-tax differences. My empirical evidence indicates that ATS mitigate such mispricing, with pricing errors being lower among firms acquiring ATS compared with firms without ATS. Collectively, these results support the notion that ATS improve audit quality through knowledge spillover. Moreover, the improved earnings quality among firms acquiring ATS in turn helps reduce investors’ mispricing of book-tax differences.