For Senior Technical Assistant (Electronics) you need B. Sc in Electronics or Three years Diploma in Electronics Engineering from recognized University/ Institute. The recruitment notification or the link given therein would be assessing your suitability for the post as also your knowledge of your discipline. I doubt there is a book for it, other than your textbooks, for handling technical questions during written test/ interview that forms a part of the selection process. However, you may search for suitable books at the bookstores stocking books for competitive exams.