According to James Maleszewski, author of the blog Grognardia (far more knowledgeable and prolific than I am), there is a perfect RPG book, and it is:
The Traveller Book.
It has been my Traveller rules-of-reference since I got my copy as a gift from and old friend. Mine doesn't have the dust jacket, and I've penciled in lots of house rules, and stuck in rules/ideas printed from other sources. It is the book I turn to when I need to know the order of results on the Rumor Table or what small craft a Type-C cruiser has aboard. I admit that after 30 years, I've got so much of the rules memorized (or think I do) that I don't have to consult it except for details like table entries. I even wrote an index for it once, just for practice.
Read James' explanation here.
Image borrowed from Wayne's Books.