ATTN: As of May 2016, there is a new MLA Handbook (the 8th edition)!
"Which edition should I use?"
The new 8th edition was published in May of 2016. It will take time before the new standards are fully adopted, so you should Check with your instructor to find out which edition they want you to use.
"How are the 7th and 8th editions different?"
If you are familiar with the 7th edition, you may want to read about the changes you'll find in the 8th edition at Purdue University's Online Writing Lab page:
What's new in MLA 8 (recommended for tutors & instructors; 1/2 hour online video from NoodleTools)
From the Modern Language Association
Great videos for learning MLA 8
The following sites allow you to automatically create formatted citations and to store and manage your citations. Be aware that these citation tools provide basic functions at no charge; advanced tools may be available only for a fee.
NOTE: It's advisable to check automatically generated citations. They may contain inaccuracies which you need to correct.
Citing ebooks (or digital files) is discussed broadly in the MLA Handbook for Writers of Research Papers, 7th edition. (section 5.7.18.) Here is a useful online guide which might help clarify how to cite an ebook you have accessed on a Kindle or other ebook reader: