I too had credit problems. It was holding me back from buying a house, or investing. (My interest is wholesaling to start). I had a disastrous and expensive experience with a repair service (recommended by an investor's association, btw). After that I decided to do some studying on my own about how to improve my score.
I found a book called "Guaranteed Credit" by Arnold S. Goldstein (be sure to use the middle initial when doing a search!). The book I have is a 1995 edition. I think there is a newer one. It is easy to follow and has numerous sample letters to creditors, the credit bureaus, etc. I was able to get my credit score up significantly, to 720, and qualify for a mortgage in less than a year. It's been a long time so I don't remember what my score was to start with, but it wasn't good.
Arnold S. Goldstein has some other books which I haven't read, but which I bet are good...dealing with the IRS, asset protection, more. You can find "Guaranteed Credit" at www.Half.Com for less than $5 including shipping, and even the 1995 version should be of a lot of help because of the links it has to updated sites such as the Fair Credit Reporting Act. Addresses to the credit bureaus are probably out of date since they keep changing, but you can find them elsewhere, of course. Repairing my own credit this way cost me only the price of the book and Certified Mail postage...and it was "private."