Reddit mentions of Indestructible Truth: The Living Spirituality of Tibetan Buddhism (World of Tibetan Buddhism, Vol. 1)

Two of my favorite books on the topic are very in-depth and detailed:

• Indestructible Truth: The Living Spirituality of Tibetan Buddhism (World of Tibetan Buddhism, Vol. 1)
  
  
• Secret of the Vajra World: The Tantric Buddhism of Tibet (World of Tibetan Buddhism, Vol. 2)
  
  I seriously can't recommend them enough. I think they are brilliant. They are more academic than "scriptural" if that makes sense. They're the sort of books you'd read in college that are about Tibetan Buddhism.

This may help:

http://www.berzinarchives.com/web/en/archives/study/comparison_buddhist_traditions/tibetan_traditions/intro_compar_5_traditions_buddhism_bon.html

I can also recommend three comprehensive books:

http://www.amazon.com/Indo-Tibetan-Buddhism-Buddhists-Tibetan-Successors/dp/1570629730

http://www.amazon.com/Indestructible-Truth-Spirituality-Tibetan-Buddhism/dp/1570629102/ref=sr_1_2?ie=UTF8&qid=1321049941&sr=8-2

http://www.amazon.com/Secret-Vajra-World-Tantric-Buddhism/dp/157062917X/ref=sr_1_3?ie=UTF8&qid=1321049984&sr=8-3

Good luck!

For a good overview of the four schools of Tibetan Buddhism, try this one.
[Indestructible Truth - The Living Spirituality of Tibetan Buddhism Vol 1, by Reginald Ray]
(http://www.amazon.com/Indestructible-Truth-Spirituality-Tibetan-Buddhism/dp/1570629102?ie=UTF8&keywords=indestructibe%20truth&qid=1465296734&ref_=sr_1_sc_1&s=books&sr=1-1-spell)

Part 2 within Vol 1 of this book, delves on this topic.