Hydrology is a vast area, therefore books and journals have been published on different aspects and areas of hydrology, e.g. physical hydrology, hydrogeology, vadose zone hydrology, groundwater, remote sensing in hydrology, soil hydrology, hydrological modelling, water quality, etc. Therefore, it would be easier if you could narrow down your search for particular areas in hydrology to find the best resources which could cater your requirement.
Few books on hydrology in general:
Physical Hydrology - S. Lawrence Dingman
Hydrology - Brutsaert Wilfried
Hydrology: principles and processes - M. Robinson, RC Ward
If you are looking for hydro-logical data and information (current and historical) of Sri Lanka, you may refer into 'Hydrological Annual' published by the Irrigation Department, Sri Lanka. That can be downloaded from the the Irrigation Department website (irrigation.gov.lk) under the publications.