One day, you may be able to travel with firearms and not worry about violating laws in other states. But until that day comes, you face a patchwork of confusing rules every time you cross a state line. You risk harassment, fines, confiscation, or even arrest. Why take the chance? With The Traveler's Guide to the Firearm Laws of the Fifty States you can travel with confidence.

Written by Kentucky lawyer and firearm law expert J. Scott Kappas, this essential reference explains all the basics for armed travel, including concealed carry laws, where and how to store guns in your vehicle, weapon types allowed, where open carry is legal, and more.

This portable easy-reference guide gives you ...

A state-by-state summary of vital gun laws that will affect you as you travel.
A handy "Traveler's Checklist" for each state, including information on permits, rules on self-defense, concealed carry, open carry, and vehicle carry.
Definitions of frequently used legal terms, such as "Castle Doctrine," "affirmative defense," and "loaded" gun, which doesn't always mean what you think.
Advice for hunters, collectors, and competitors who travel with long guns.
The restrictions on assault weapons and machine gun ownership.
A complete CCW national reciprocity list, including a map you can personalize to show where your concealed carry license is valid.
A list of each state's Attorney General and where to find the definitive gun laws for any state in the U.S.
A special firearm law appendix with bonus information for travel in Canada and Mexico.
A state-by-state ranking on a scale of 0 to 100 for relative gun law freedom around the country.
If you own or carry a gun, don't dare travel out of your state without The Traveler's Guide to the Firearm Laws of the Fifty States. Read the book before you travel. Pack it with your gun gear. Keep a copy in your vehicle. Use it as a reference so you'll know what to expect when you approach a state line.

Quoted:
For Android phone users, I would also recommend CCW – Concealed Carry 50 State

They update it fairly often.  I'm sure there is one for iPhones, but it is probably expensive and not as good.