Here are some of the best books for GATE 2015, though it must be said that you have to invest a significant amount of time in these books to get some thing good out of it. These text books can be used as a standard reference, when ever you have a doubt you can go through it. Reading is a good habit, takes time to develop but is essential to learning.
Electrical Machinery by P.S Bhimbra
Although a fine books but it can be a bit intimidating at the beginning, Plenty of examples have been given in between the chapters so no worries there. Solution manual not available for end of the chapter problems.
The above given books is a good read, though a few GATE topics are not covered, solution manual for this books is available online which will help in self preparation.
Signal and Systems
You can close your eyes and go for this signals and systems book, topics are explained in very easy manner and complete solution manual is available online, good books for self preparation.
The above is a standard textbook in control system and its complete solution manual is available online, this textbook is little hard to understand but it has a good set of practice problem.
This textbook by Nise is very easy to understand and concepts are explained in a very lucid way. On the negative side questions given in practice problems are more suited for the mechanical students. This books complete solution manual is available online
A good books as a reference for measurement but its physically huge size is a negative, very few explanation problems and no solution manual for the end of the chapter problems.
Although the above is a standard textbooks it is little difficult to understand. Its complete solution manual is available online.