Best study tools on the net or books
Piya Tan at http://dharmafarer.org has translations and commentary of many of the suttas. Using the "sitemap" on the right or bottom of the page, you can find specific suttas (sorted by number), or, his "sutta discovery (SD) series" which is sorted thematically.

Some people disagree with [some of] his commentary (see e.g. this topic).