The last I heard, the fire at the library was most likely due to a mechanical malfunction and was not intentionally caused--whether that is definitive or not remains to be seen but I know the police are being careful not to speculate in any way. As for 'other explosions', the police have repeatedly said that there were only two devices at the scene and no others so rumors about others apparently were just rumors.
Boston Marathon bombs placed in pressure cookers - ESPN Boston
Boston Marathon bombs placed in pressure cookers - ESPN Boston