Police said a library user used the system to steal and sell thousands of books bought with taxpayer money.
According to court papers, 33-year-old Terrance Mitchell spent hours at library computers logged on to the book-buying website Cash4Books.
He would find what was going for a good price, like cookbooks and auto manuals.
Then, he'd find the book on the shelf and peel off the bar codes.
"It's really despicable what this person was doing." Said Sno-Isle Communications Director Ken Harvey.
A website employee alerted a manager at Sno-Isle, which runs libraries in Snohomish and Island Counties.
A shipment of books sold by Mitchell included one form the library system.
Staff went to find the book on their shelf and found it missing.
Staff spent several more hours checking the full list of books Mitchell sold online.
In all, 2,471 books disappeared from the Lynnwood, Mountlake Terrace and Edmonds libraries in just three months.
The books cost $9,000 in taxpayer dollars to buy; police said Mitchell made $1,100 selling them.
"It's crazy. The guy's a bum, obviously. I mean, someone coming in to steal from the public, really," said Greg Lynch, who uses the library to check out children's books for his sons and resource materials to read on the bus to work.
"Books are expensive. I can't buy a book every 2-3 weeks when I finish one on the bus," said Lynch.
Police arrested Mitchell last month and said they found drugs and fake IDs. They said Mitchell told officers more people are involved.
Library staff said they have recovered 100 books so far.