"In my whole life, I have known no wise people who didn't read all the time -- none, zero. You'd be amazed at how much Warren reads -- and at how much I read."

That's a Charlie Munger quote (Munger is Warren Buffett's right-hand man at Berkshire Hathaway (NYSE: BRK-B)). There is little question in my mind that there are few if any activities that can help you improve as a person, and an investor, more than reading. Here are two investor letters that Buffett reads as soon as they are released:

Jamie Dimon's shareholder letter
On Friday, Buffett told Bloomberg that "Jamie Dimon is a fabulous banker, and probably writes the best annual report in America; I grab his report when it comes in and my friends do, too."

We'd certainly expect Buffett to read Dimon's letter if Berkshire were a JPMorgan Chase (NYSE: JPM) shareholder, but it isn't (per the most recent filings). Buffett must believe that he can glean insights about the economy and the banking industry from reading Dimon's letter. Buffett takes an interest in banking; Berkshire is the largest shareholder (by a wide margin) of JPMorgan competitor Wells Fargo (NYSE: WFC). At 39 pages, Dimon's latest letter is comprehensive, covering themes ranging from the role of banks in the postcrisis world to the European sovereign debt crisis.

Howard Marks' memos from the chairman
"When I see memos from Howard Marks in my mail, they're the first thing I open and read. I always learn something, and that goes double for his book," Buffett has said.

You've probably never heard of Howard Marks, the founder and chairman of Oaktree, a value-oriented fund manager. Marks and his firm, which focuses on debt and real estate investments, are highly respected -- and highly successful. Savvy investors, including Buffett, have long enjoyed his memos for his insights into risk, successful investing, and the investing environment. You can, too -- they're available to all on Oaktree's website. As you can verify -- the memos go back to 1990 -- Marks has been prescient in warning investors about forming asset bubbles (note that his latest memo is titled "How Quickly They Forget").

An efficient way to capture the essence of Marks' thinking is to read his book, The Most Important Thing: Uncommon Sense for the Thoughtful Investor. If you enjoyed Nassim Taleb's Fooled by Randomness or The Black Swan, you're certain to enjoy it; I have my copy, and I think all investors should read it.

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Fool contributor Alex Dumortier holds no position in any company mentioned. Click here to see his holdings and a short bio. You can follow him on Twitter. The Motley Fool owns shares of Berkshire Hathaway and JPMorgan Chase. The Fool owns shares of and has created a ratio put spread position on Wells Fargo. Motley Fool newsletter services have recommended buying shares of Berkshire Hathaway. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.