Hello candidate,

Here is the list of some of the best books for  mechanical engineering for GATE preparation-

Strength of Materials - SS Rattan.

Theory of Machines - RS Khurmi.

Mechanical Vibration - GK Grover.

Design Of Machine Elements - VB Bhandari.

Engineering Mechanics - SS Bhavikatti.

Fluid Mechanics - RK Bansal.

You can find these books easily in PDF format on platforms such as Kindle,etc. Hope you found it helpful!!