The 31-year-old made over 300 appearances during almost nine years with Royals and was a regular under former managers Alan Pardew and Steve Coppell.
He was also a mainstay of the all-conquering side which won the Championship title with a record 106 points in 2005/06, securing the club's first ever promotion to the top flight.
He became a permanent fixture in the Premier League alongside the likes of Fulham and ex-Arsenal midfielder Steve Sidwell.
Harper, a close personal friend of Crusaders boss Bobby Wilkinson, eventually left the Madejski Stadium in September 2009 to join Sheffield United where he spent a season before joining Championship side Hull City.
The ex-Gunner was released by Hull at the end of the 2011/12 campaign having finished the season on loan at Wycombe Wanderers.
Reports suggest Harper has been attracting interest from League One Coventry City, but the players has decided to move to Bulpit Lane to be reunited with Wilkinson, who continues to have business links with the Royals.
And Wilkinson is delighted to have the midfielder on board, whether for the short or the long term.
He said: "Either way, if I have him for the first month of the season or for half the season, we will benefit from his experience. He is a great addition to the squad."
Harper was acquainted with his new Crusaders team-mates at the end of last week and made his debut on Saturday when Hungerford cruised past Reading Football League side Mortimer 9-3 in a pre-season friendly.
He was also in action for Hungerford on Tuesday night in a 3-2 home defeat by Wokingham & Emmbrook in a friendly.