You must know that History covers a major part of the General Studies subject in UPSC exams so aspirants should focus more on this subject and this is the only most confusing subject in General Studies that can not be completed easily like other subjects asked in GS sections as Economy- Exact as always, Geography - Records are Exacts, Indian Polity - Similar Processes and Environmental Studies with Science and Technology so it's good to see your interest in History Subject and this is also very good that you are using Spectrum Book to cover the part of History in GS papers. This is a very good book for all UPSC ICS aspirants because this book has the quality of providing complete details in a short manner so aspirants could learn easily and get good command over History
If you wants to use this book to get good score in examination then first of all you should read all 16 Table Contents given in the book because all data of book is according to the mentioned table of contents so you should read then sort according to the requirement means start reading all those topics which are major and mostly asked in examination and mainly you will find out only the History of Modern India so there is no chance to get in depth and just complete the entire book because nothing is irrelevant in that book so you have to cover all 16 parts of this book on time
History is the subject that should be completed in sequence so you should not leave sequence of any topic and keep your reading and revision continue and try to link each topic with other topic that you have already completed and also with upcoming topic because most of the major parts in this subject are related to each others as if you are studying about kings then you will see that a Genus starts at the time and ends on time so you have to read previous years question papers first then see that what are the parts that are mostly asked in examination from History section then take help of this book in completing those topics and as I have already mentioned that nothing is waste in that book so you have to make a target to read this book more than 8 times before preliminary examination and 15 times before main examination in description
There are some major topics that are official and frequently asked in Indian Civil Service Examination and all questions of History subject are set from those topics so you should concentrate on all those topics and keep reading continuously by using some other books because spectrum provides only modern history but Ancient and Medieval parts are also very important so you should cover all parts of this history by reading these good books :
NCERT Books for History - +2 Level
Bipin Chandra - History of India
Romila Thapar - Ancient History of India
AC Banarjee
Gopal Krishna
All details about History subject are mentioned in attached notification means if you choose History as your optional subject then also you should read this advertisement to get full syllabus and prepare well