If you’re indiscriminate on asset class, a few books I would suggest:
Options, Futures, and Other Derivatives by John Hull is a great starting point
The Handbook of Fixed Income Securities by Frank Fabozzi is good if you’re interested specifically in fixed income.
Heard on The Street by Timothy Falcon Crack for more technical interview questions.
Come Into My Trading Room: A Complete Guide to Trading by Alexander Elder
I would also recommend reading financial news on a daily basis, including Financial Times, Wall Street Journal etc. to get a good sense of the markets and formulate your own ideas.