OBJECTIVE: Book Prescription Wales (BPW) is a pilot bibiliotherapy scheme launched in July 2005 as a primary care treatment option for people with mild to moderate mental health problems. In an innovative model, patients are prescribed self-help books from a list, to borrow from local libraries. Our objective was to evaluate its implementation, focusing on the issue of equity of service delivery. METHODS: Data were gathered from Welsh Assembly Government concerning project set-up and borrowing rates. Mailed questionnaires were completed by 21/22 (95.4%) Local Health Boards and 44/64 (68.8%) Community Mental Health Teams. In addition, 327 out of 497 (66%) primary care practices were surveyed by telephone, 20 prescribers took part in in-depth telephone interviews and three focus groups were conducted with library staff. RESULTS: From July 2005-March 2006, books were borrowed 15,236 times. There was a 10-fold variation in borrowing rates across local authorities (1.07 to 10.18 loans/1000 people). The priority which Local Health Board staff reported giving to the scheme varied. Uptake among prescribers was mixed: in 35% of general practices (n = 116) no-one participated. Prescribers reported different ways of using the bibliotherapy scheme. Library staff reported issues of patchy uptake. CONCLUSION: Variation in usage of bibliotherapy raises questions about equity; it is unlikely to reflect the distribution of people who could potentially benefit. Factors influencing variation existed all along the implementation chain. It is not always possibly to separate demand-side and supply-side factors when considering equity and service innovation in health care.