Assuming you have one more year to prepare for your placements, this might help
One of my seniors replied this when I asked the same question:
Read the following carefully. they should help
1. Great book of puzzles and teasers (helpful for tests)
2. How to ace the brain teaser interview (try and read all of them, direct questions are asked in interviews/tests from this book)
3. Heard on the street (for probability based problems)
4. Vault guide for finance (brainteasers section only)
5. fifty challenging problems in prob
6. 40 puzzles and problems in probability
7. Nicks mathematical puzzles (not all, only those which make sense to you , random problems di hui hain, i didnt read it, but Finmechanics asked direct questions from this book in their test)
book 5 and 6 u can read at the end, start doing book 1 and book 2 aur baki dekho bass time milne pe, sending you some links as well, for brainteasers