This appears to be Wizard's Bane, by Rick Cook. It seems that neither it nor any other novel of Cook's are currently available in the Baen Free Library, but Baen still sells ebooks of the novel, as well as its four sequels, and you can read sample chapters on the Baen website.
As noted in the comments, Cook has been unable to continue the series due to his health. The sixth book, The Wizardry Capitalized, remains unfinished , but Cook released it freely to read online.
...one of their most powerful members cast a spell to bring forth a
mighty wizard to aid their cause. What the spell delivered was master
hacker Walter "Wiz" Zumwalt. The wizard who cast the spell was dead
and nobody— not the elves, not the dwarves, not even the dragons—could
figure out what the shanghaied computer nerd was good for.
But spells are a lot like computer programs, and, in spite of the
Wiz's unprepossessing appearance, he was going to defeat the
all-powerful Black League, win the love of a beautiful red-haired
witch, and prove that when it comes to spells and sorcery, nobody but
nobody can beat a Silicon Valley computer geek!