For me, it's not whether the sentence is too complicated or too long, but the tools you use within it to make it stick (not the best word but I can't think of a better one at the moment) in the readers or listeners mind.

One of my favorite classes in uni was rhetoric, and one of my favorite books to this day is Farnsworth's Classical English Rhetoric. I highly suggest it as a tote around, open to a random page and read as much as you like and never be disappointed reading.

http://www.amazon.com/Farnsworths-Classical-English-Rhetoric...

There are quite a few at the top of this list, and they vary based on subject. Are you speaking about technical books, or literature, or something else?

Regardless, the one that I spent a good year reading through multiple times was Farnsworth's Classical English Rhetoric. A brilliant book that challenged my knowledge of rhetoric and enabled me to hone my rhetorical skills. You can read it to learn or read it just for the awesomeness of the passages.

http://www.amazon.com/Farnsworths-Classical-English-Rhetoric...