If someone wants to seriously dive in, here is my *top* recommendation:
1) Read the LDS manual "Gospel Principles" (https://www.lds.org/manual/gospel-principles?lang=eng), and then also read Rob Bowman's corresponding review articles (http://mit.irr.org/gospel-principles-archive). He is top-notch and there is enough meat there to keep one chewing for a long time. As Bowman writes, "[Gospel Principles] provides the best overview of LDS doctrine and values and is arguably the religion’s most important and enduring curriculum publication." My recommendation is to let Bowman's materials at IRR.org be your first go-to for quality online material.
2) I would also highly recommend the materials of my own colleagues, Bill McKeever and Eric Johnson, at mrm.org
3) UTLM: http://utlm.org/navtopicalindex.htm
4) http://mormonthink.com/ has great historical material, but comes from an agnostic/liberal perspective.
5) If someone wants to memorize verses in preparation for conversational or street evangelism to Mormons, see James White's list:
http://vintage.aomin.org/MEMVER.html (this helped me a ton when I moved out [to Utah])
6) For those needing introduction to terminology differences:
7) For a "differences" chart see the front of mormoninfo.org
8) For great introductory videos done by a Christian: http://www.ldsvideo.org/p/video-index.html
Aaron's own stuff:
- The conversation-starting tract I most often use: https://aaronshaf.files.wordpress.com/2016/01/authority-tract.pdf
- The shock-and-awe video I use to help Christians see how stark the differences are between Mormonism and Christianity: https://www.youtube.com/watch?v=WyH61ybnPBE
- My evangelism to Mormons 101 presentation notes: https://docs.google.com/document/d/1gvncU6tpI3MSUAUO6a1kpLWDMqrOqiPSrwiPZzpwB3g/edit?usp=sharing
- This is more subtle, but "Matthew for Mormons" aims to share the gospel via sharing Jesus-stories from Matthew: https://www.youtube.com/watch?v=2WbZU9vfEr0
- My YouTube channel has quite a bit of related presentations and ex-Mormon Christian testimonies: https://www.youtube.com/jesusnotjoseph]
More Resources on LDS/Mormonism from our own website.