A Saugus woman who stole more than $850,000 from a library where she worked pleaded guilty to several charges Friday, said US Attorney Carmen M. Ortiz’s office. Linda E. Duffy, 65, was charged with four counts of mail fraud, 10 counts of money laundering, and aggravated identity theft, Ortiz’s office said. Beginning in 2004, Duffy diverted charitable donations and other payments from the Saugus Public Library to her personal account, the statement said. Duffy was forced to resign in 2011, according to Ortiz’s office. Ortiz’s office said she could face up to 20 years in prison.