I have a question for you pros. What single reference book (preferably still in print) would you have if you were only allowed one single volume to design and build the systems you install. Not just hydronic theory; that probably becomes intuitive after years in the business; but all those day-to-day details that nobody can remember all of. You know, head losses through different pipe sizes and fittings, GPM to ft./sec conversions and so on. And since, inevitably, no one book can do it all, what 2 books would you need to keep yourself confident when you need to come up with a number for comparing two different system options. I consider books to be tools and don't need much of an excuse to buy another tool. Price isn't even an issue here but avoiding a lot of library duplication. Simple title and author would suffice. I think a lot of the readers of this forum would be interested to see if a concensus forms around a few good books.