Porting to BeOS
This is a book written in 1997 by Martin "MC" Brown. He has now released a pdf version of the book. If you'd like a dead tree version of the book you can still find it on Amazon and others, either new or used. Here's the pdf of the book. MC is still active writing books, here is his blog: planet.mcslp.com
This book contains a lot of information, some of it a bit dated, but much of it still applies to porting apps to Haiku.