Mannu, it is good to see that you want to crack the RULET Examination on your own through self study without coaching and hence you require the best study materials for preparation. Now, try and understand that the syllabus of RULET is very similar to other law entrance exams apart from a few new topics like General Science, Social Studies etc. so some of the books which are used for preparation of other law entrance exams like AILET would also suffice for your praparation. So,here are a few books which you can refer to for your preparation:--

WORD POWER MADE EASY by NORMAN LEWIS

LEGAL AWARENESS AND LEGAL REASONING by AP BHARDWAJ

RULET 2020 by CHANDRESH AGARWAL

PROFICIENCY IN READING COMPREHENSION by AJAY SINGH

LAW OF TORTS by RK BANGIA

MP JAIN INDIAN CONSTITUTIONAL LAW by MP JAIN

LUCENT GENERAL KNOWLEDGE

ENCYCLOPEDIA OF GENERAL SCIENCE by various authors

Apart from these books, you should refer to the previous 5-10 years of RULET question papers too and try and give mock tests as well to have a check on your preparation level and try and analyse those mocks too so that you can work on the areas where you are facing any problem and reduce the chances of mistakes.