Here's a quick tip. Don't be like many job seekers that treat cover letters as an afterthought or don't bother with them at all. If you include a cover letter with each resume you send, you'll have an advantage that could make all the difference in determining whether you get an interview. But before you write your letter, consider this:
Q: Someone told me that hiring manager don't bother reading cover letters, so why should I bother?
A: Managers and HR professionals at the best companies invest time in making sure they hire the best and so they will take the time to carefully read your entire application. And you do want to work with a good employer right? If a hiring manager cuts corners and hires nearly anyone then imagine what your coworkers would be like if you decided to work at that sort of organization.
The only reason a good hiring authority will not read a cover letter is if a quick initial scan indicates that the cover letter is weak and doesn't have any value to add to your resume. So make sure your letter is well written!
Another possibility is that a busy hiring managers might not read cover letters on the initial screening. But they will read it on the second pass when considering the short list.
Overall, including a cover letter can help you, and it definitely will not harm your chances at and interview.