Some good questions there and there will be numerous different opinions on it.
Regarding BBQ bible I would go for Meathead - The Science of Great Barbeque and Grilling by Meathead Goldwyn. Available on Amazon.
Ref first BBQ that is a little more debatable and would also depend very much on budget. I have a WSM 47 for smoking and love it. However, I wouldn't want to use it for grilling as it would be a real faff and the grill would be small and low to the ground.
I also have a couple of Weber Grills. The latest version is the 57cm master touch. These are great bits of kit for grilling and perfect for a beginner. Your hubby would be able to start smoking low and slow on one of these too. His options for smoking may be a little limited using this but he would be able to do enough on it to be able to tell whether he wanted to do more. At that stage you could look at a separate WSM smoker.
Finally, if money is no object and you really do want 1 piece of kit to do low and slow and hot and fast, then look at a Kamado style BBQ. Something like a large Big Green Egg. This will set you back over £1k however once you're all set up with it
One final bit of advice - if he's going to be doing low and slow cooking invest in a remote digital thermometer and probe. The analogue thermometers that are built into the lids of these BBQ's are so inaccurate as to be useless, and If you're wanting to keep the BBQ temp between 225 & 250f for 4 to 18 hours (depending on what you're cooking) you need peace of mind that the BBQ temp is stable. I have a Maverick ET 772 which is very popular. I think this has been replaced by the 773 now.