Okay, now for a couple of specific issues:
Tax Records (U.S. version)
It is a commonly held belief that the IRS requires you to keep your tax records for seven years. Actually, according to the records manager of a company I used to work for, that's not quite the case. According to her, the IRS regulations require you to keep your records for three years. However, if they do decide to audit you, they can go back seven years. And since no one would want to depend on an adversary for records concerning his or her own interests, everyone just automatically keeps seven years' worth of records. And just so we're clear, that means not only your tax filings, forms, and schedules, but also all of the supporting documentation: receipts, 1099 forms, spreadsheets, QuickBooks reports, e-mails that pertain to the taxes, and anything that would be necessary to defend deductions or interpretations, most especially any communications from the IRS. In this area, it's better to err on the side of caution; if you're not certain you need to keep it, you should probably keep it in the file.
Contracts (U.S. version)
Every state in the U S has regulations that define certain types of records which businesses must keep, even self-employed businesses like writers. As long as you as a writer are a one-person shop, most of them won't be an issue. If you get to the point, however, where you are paying people to perform business functions for you (accountant, secretary, researcher, etc.) then you need to educate yourself on what your state requires.
There is one type of business record retention about which even the one-person writer shop needs to know, and that is your contracts and agreements. Almost every publishing contract between an author and a publisher or a publishing platform will contain a clause that says that in the event of disagreement between the parties, the contract is to be interpreted under the laws of a certain state. Most of the traditional publishing contracts indicate they will be interpreted under the laws of New York.
Obviously you want to keep the contract or agreement as long as it is active; in other words, as long as there are obligations between you and the other party which must be observed or performed.
But at such point in time as the contract has basically terminated—all parties no longer owe anything to anyone under its provisions—what do you do with it then?
Hint: don't throw it away.
Every state has statutes or regulations that stipulate how long such a terminated contract must be retained by the parties subject to it. Here's the summary: if you or your publisher reside or work in Louisiana, or if the contract says it will be interpreted by the laws of Louisiana, the rule is to hold it fifteen years past termination. All the other states have settled on a term of five years.
In states other than Louisiana, the only caveat I would raise would be if the contract had provisions that dealt with finances, you should probably keep it until the last year it operated has passed its seventh year tax retention.
And finally, the contract file should contain anything that would have a bearing on the intent of the parties in drafting the agreement, as well as anything that might bear on how it should be interpreted. So yes, you may need to keep some letters or e-mails to support that contract.
In summary: be organized, back everything up to protect yourself, and manage your records.