Stranger in a Strange Land
(4) Base Camp
To prevent some frustration, you'll need to know two things before tackling Stranger in a Strange Land head on:
(1) Heinlein writes about some pretty tricky social and cultural concepts. Naturally, this can result in language that can get a little tricky, too. It can take a while before you grok what it means to grok, and occasionally Heinlein will throw down a gauntlet of a concept like "Apollonian vs. Dionysian cultures." Yikes. Although the characters will usually explain the concepts in context with what's happening, some outside research might be necessary. Read safe and have Google at the ready.
(2) Heinlein writes this novel from a third-person omniscient perspective, meaning he will freely jump between characters' perspectives and even sometimes pull back to give us a cosmic view of events (as opposed to, say, Ender's Game which is written solely from Ender's perspective). Heinlein switches character perspectives without warning and occasionally multiple times in a single chapter, so be prepared for a slight adjustment period. But once you get into the groove of Heinlein's pacing and style, the climb will be exhilarating without being debilitating.