Can I write off a high-end agenda cover that I use for my self-employment business? The agenda cost around $800 and I use it to track invoices, billing, and my calendar.

[JohnW15]

This is a tricky issue, and you'd really need to use your own best judgment.  You can almost surely claim at least some of the expense, but the "high-end" description of the agenda cover muddies the waters.

According to "What Can I Deduct?" on page 3 of IRS Publication 535, Business Expenses,

To be deductible, a business expense must be both  ordinary  and  necessary.  An  ordinary  expense is one that is common and accepted in your industry. A necessary expense is one that is helpful and appropriate for your trade or business.  An  expense  does  not  have  to  be  indispensable to be considered necessary.

So, was it "ordinary and necessary" to buy a high-end cover?  It might be, if others in your line of work used something similar -- after all, presenting yourself well with clients has value. 

Think of it this way -- if audited, could you justify the expense to the IRS, as opposed to a less-pricey version?  If the IRS found that a less costly version would have sufficed, they'd probably allow a deduction for the price of the less costly version.

Just use your best judgment -- that's all you can do.