I sometimes enjoy roaming through used bookstores, buying old handbooks for a pittance. There is nothing singular about most of these books. In building your own library, take what appeals to you. Such a style guide should not just be any reference, but one of considerable depth and standing. Some dictionaries include extensive guides to usage, style, orthography, etc.
Some large companies publish their own style guides.
I recommend sticking with a published standard, unless you happen to be experts in the field, and in the business of publishing writing handbooks. It is especially interesting to consult references that are 50 or 100 years old, to see how a usage may have evolved, or, even more interestingly, to see how practices go in and out of fashion. As with the Periodic Table, or the physical energy spectrum, or the ratios of trigonometry, the genuine basics don't change. All others are hardcovers. Glazier, Stephen, The Random House Word Menu , Random House, 1992.