A bomb exploded in a Left Bank book shop here this evening, injuring four persons, in the latest of a series of mysterious attacks on Parisian landmarks.

Last night, eight persons were injured when a bomb exploded at a shopping gallery in the Champs Elysees. An hour later, a time bomb was discovered in a public lavatory at the top of the 985-foot Eiffel Tower, but was defused before it could explode.

In a letter to the French news agency, Agence France-Presse, a previously unknown group calling itself "The Committee for Solidarity with Arab Political Prisoners" asserted responsibility for the Champs Elysees bomb. Police said that they did not know whether the letter, which demanded the release of several persons convicted of terrorist crimes, was authentic.

Tonight's explosion in the basement of the Gilbert-Jaune bookshop in Place St. Michel was followed by a fire that devastated all three stories of the bookshop. The bookshop, opposite the cathedral of Notre Dame, is one of the largest and best known in Paris.

The blast rained glass over the surrounding square, which was cordoned off by police.

Police said it was too early to establish a link between the three bombs. Initially, they looked into a theory that the Champs Elysees explosion could have been the work of criminal racketeers seeking protection money from local shopkeepers.

Among the prisoners whose release was demanded by the group that said it was responsible for the Champs Elysees bomb was Georges Ibrahim Abdallah, the alleged leader of the Lebanese Armed Revolutionary Factions. Abdallah has been charged with complicity in the 1982 murders in Paris of Lt. Col. Charles Ray, deputy U.S. military attache, and Yacov Barsimantov, second secretary at the Israeli Embassy here.

The other prisoners were Waroujan Garbidjian, leader of the Armenian Secret Army for the Liberation of Armenia who is serving a life sentence for his role in a bombing at Orly airport in 1983 which killed eight persons, and Anis Naccache, leader of a group of Iranian terrorists that tried unsuccessfully to murder former Iranian prime minister Shapour Bakhtiar in July 1980.

Various groups have asserted responsibility for terrorist acts in France during the past few months including the Islamic Jihad and a leftist group known as Direct Action which is believed to be connected with the Red Army Faction in West Germany. Police said that crude incendiary bombs, which exploded in Paris department stores before Christmas, were placed by a crank.

An explosion at the Eiffel Tower was avoided because an employe found the suspicious package shortly before closing time last night. The timing device, with about seven pounds of explosive, was set to go off an hour after the tower closed at midnight.

Built by Gustave Eiffel for the universal exhibition in 1889, the tower is one of the most popular tourist attractions in France, drawing more than 3 million visitors each year.