Billionaire investor Carl Icahn continued his epistolary shouting match with Yahoo chairman Roy Bostock on Monday with a letter in which he urges Bostock to justify his director compensation by releasing his time sheets and accuses him of purposefully not answering questions.

What's clear from the tone and frequency of Icahn's letters is that he seems more and more convinced that, as a Yahoo investor, he needs to push his slate of candidates to unseat the company's directors at the next shareholder meeting in August and, he hopes, bring Microsoft back to the negotiating table.

Separately on Monday, Yahoo announced the filing of its Definitive Proxy Statement with the US Securities and Exchange Commission, in which the company, unsurprisingly, urges shareholders to re-elect the current board and reject Icahn's candidates.

Calling the upcoming meeting on August 1 "the most important for stockholders in our history," Bostock and CEO Jerry Yang said in the proxy statement that the board and management's strategy "to create value ... is gaining traction."

The focus of the latest angry exchange of letters, which started last week, is Yahoo's adoption of an employee severance plan that both Icahn and Yahoo shareholders suing the company allege was implemented to sabotage Microsoft's attempt to acquire Yahoo.

Bostock and Yahoo's top executives maintain that the severance plan was necessary in order to retain employees in light of the uncertainty created by Microsoft's pursuit of Yahoo, which officially ended after three months in early May.

"I cannot understand why the Yahoo board feels so strongly about its 'poison pill' severance plan and why it continues to refuse to rescind it. How can you continue to repeat that your severance plan is in the best interests of shareholders and employees?" Icahn wrote in Monday's letter.

Yahoo didn't immediately respond to a request for comment, but on Friday issued a brief statement saying that Icahn, in his previous missive, had inaccurately interpreted the "retention" plan and that his suggestions that the plan be cancelled would have a "destabilizing impact" on the company. Yahoo also accused Icahn of having "no credible plan to operate" the company.

The brevity and content of Yahoo's statement on Friday clearly irked Icahn, who ripped into Bostock on Monday.

"In your press release from Friday, you stated again that I do not have a credible plan for Yahoo. Did you even bother to read my letter, which went into great detail on what measures I would ask the new board to take? Ironically, while you keep inquiring about my plans, it is interesting to note that Yahoo's board has been busy reaping great compensation benefits. Indeed, you made approximately $10,000 per week last year -- not bad for a board member. I believe most of your shareholders would be interested in seeing your time sheets -- especially in light of the fact that, in my estimation, most of your so-called 'plans' over the last few years have been failures," Icahn wrote.

Icahn also reiterates his call for removing Yahoo co-founder Jerry Yang as CEO and returning his "Chief Yahoo" title, so that the company can hire "a talented and experienced" replacement, offering Google CEO Eric Schmidt as a model.