19 CFR, readable

The regulations are the job, and the official readers make them slow. This one is built for the way customs people actually use Title 19:

• Citation jump. Type 111.28, or 19 CFR 152.103(b), and you are there. No tree-walking.
• Full-text search across the whole title, ranked, with the citation on every hit.
• The table of contents when you want to read a part end to end, and find-in-part when you know the word you are hunting.
• Bookmarks on any section. Sign in and they follow you from desk to phone.
• A pinned, dated edition. The reader tells you which edition you are reading and when it was current, on every page.

There is also an exam-faithful mode that turns search off and makes you navigate the way CBP's exam platform does. You will never need it, but the candidates you mentor might.