which book is best for ENGINEERING MECHANICS for GATE MECHANICAL?
You can refer to these books for preparation for "Fluid Mechanics & Hydraulics".
A Textbook of Fluid Mechanics and Hydraulic Machines by R.K. Bansal.
Fluid Mechanics (SIE) by Frank White.
Hydraulics and Fluid Mechanics Including Hydraulics Machines by P. N. Modi.
1000 Solved Problems in Fluid Mechanics by K Subramanya.