Hi Aryan,

The best book that you can refer is "Study Guide: BHEL Engineer Trainees Electrical Engineering" by GKP. It includes technical as well as non-technical questions along with the answers. 

As far as the syllabus is concerned the Exam is divided into two sections: Aptitude and Technical part.

*For Aptitude, the syllabus is as follows:-

Aptitude-Decimal and Fraction, Time and Work, Time and distance, Ratio and proportion, Interest concepts, Averages, Number system, Mensuration,Arithmetic operation, Graphical Analysis

Logical Reasoning- Analogy, Coding and Decoding, Directions, Clocks and Calendar, Ranking, Decision Making, Venn Diagram, Cubes and Dice, Series, Mirror Images

English- Noun, Pronoun, Adjective, Adverb, Articles, Synonym and Antonym, Prefix and Suffix, Sentence, Question Tags, Tense, Homophones, Gerund, Participle, Comprehension

General Awareness- Indian Constitution, Current Affairs, Abbreviations, Important dates/days, Financial and Economic news, Books and authors, Awards and Honors, General Politics

* For Electrical Stream, the syllabus is as follows:-

Basic concepts covering current, voltage, energy and units,capacitance, inductance etc., Circuit laws, Magnetic Circuit, AC Fundamentals, Measurement and measuring instruments, Electrical machines, Synchronous machines, Generation Transmission and distribution of energy and power, Estimation and costing, Basics of electronics which include concepts like PN Junction diodes, transistors, JFET, MOSFET etc.

All the best for the exam!

Hope you find the information relevant!