For a basic understanding of Class 12 Mathematics, I would recommend the textbook "Understanding ISC Mathematics by M.L Agrawal". It is a comprehensive text book with a plethora of solved problems and unsolved problems. What I like the most about this book is that it is very structured and problems range from easy to moderate to difficult. There is a "typical illustrative examples" at the end which is targeted for entrance exams. Below is the link to purchase this book.
There is also a textbook by Malhotra Gupta which is an excellent book for study for Class 12 ISC Maths. I would say that most of the paper setters refer to these 2 books . This book covers most of the topics in the above book. Going through a few extra problems in this book would give you a cutting edge in the ISC examination. The link to purchase this book is given below.
With the new exam pattern in the light of COVID, ISC has introduced Semester type examinations starting from 2021. For the 1st semester examination, to be held in November December 2021, I would recommend this book on MCQ'S and Case study based questions as an additional supplement to the textbook. This book has case study based questions both solved and unsolved which would be of use to the students. Also, there is a wide range of MCQ'S as well. Following is the link to purchase this book.
For those of you , who want to keep their cost low, I would recommend just purchasing this book on MCQ'S. There are a variety of MCQ questions which really make you think. This book covers the entire syllabus and the questions range from easy to difficult. Following is the link to purchase this book.
Likewise, there is a book on MCQ'S for CBSE Semester 1 exams as well. Though the topics are more or less the same, I would recommend that CBSE students stick to CBSE textbooks and ISC students do likewise. Following is the link for CBSE MCQ'S Semester 1
CBSE has Applied Math as a subject as well. The following link gives the book on MCQ'S for Applied Math.
The next two links are for the textbooks of Applied Math. There are 2 textbooks ,Volume 1 and 2. Both constitute the syllabus for Class 12 Applied Math
As a Math teacher teaching mathematics for close to 3 decades , I have created a few course for Class 12 ,which may prove useful to you.