If you’re indiscriminate on asset class, a few books I would suggest:
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Options, Futures, and Other Derivatives by John Hull is a great starting point
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The Handbook of Fixed Income Securities by Frank Fabozzi is good if you’re interested specifically in fixed income.
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Heard on The Street by Timothy Falcon Crack for more technical interview questions.
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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.