The best way for the wizard to defend a spellbook is the same as the best way for you to protect your precious computer files - have multiple backups.
But, if he hasn't had time to make a copy, if he knows someone is trying to steal the book back, he wouldn't leave the book in the shop overnight. He'd keep it on himself, likely guarded by as many guards as a 7th level wizard can afford to hire on a temporary basis.
If he must leave the book in the shop for plot points, he'd certainly have the book very well protected and secured. He's 7th level, so he knows 4th level spells. He might, for example, use Stone Shape to fabricate a 'doorless' safe in a wall or floor block. And he'd certainly know to keep the book in a lead box to block scrying and detection attempts. He could also use illusions to hide/conceal the book. And decoy books and safes to waste the thieves' time. And of course, a 1st level Alarm spell on the shop would work wonders. He might hire a dozen men at arms who hang out in a neighboring building waiting for the alarm to go off.
You say he can't use anything too damaging in terms of traps. Well, poison gas doesn't cause much physical damage and dissipates after a while. And while you say its illegal for him to create fatal traps, a) he may not care, b) bribes and Charm Person can get the well-to-do out of trouble, and c) dead thieves can't report you to the town guard. If you don't want to do that, you can always fill the shop with a Web.
Beyond defending his shop, since he knows there's likely going to be a break-in, he might have a familiar watch the shop from a distance and follow the thieves back to their home/inn/hideout. And when they aren't looking, he can rob them blind.
No.
There are 3 basic d20 roll types in 5e:
- Attack Rolls
- Ability Checks
- Saving Throws
None of these are the same and they are always called out when a roll is called for which kind of roll is called for.
So no, you don't get the bonus from Enhance Ability to attacks (or saves for that matter).
Best Answer
There is no official answer on this specifically that I'm aware of, however it's likely a part of WotC's attempts to cut down on the Wizard's versatility.
In earlier editions, especially 3.5, the Wizard had access to just about any kind of spell you could think of. This played a large part in making the "Batman Wizard" who was able to deal with just about any situation, given just a bit of time to prepare. In 4th edition, this was scaled back sharply, too far for some people's tastes, and 5e seems to be working to compromise between those two extremes.
As for why Enhance Ability specifically, it's even harder to say for sure, but I'd guess that WotC decided that, since buffing is strongly associated with Divine spellcasters like Clerics and Druids, it would make for a good place to start cutting into what the Wizard can do. Wizards still have many buffs, of course, but most tend to work more directly, either granting the target something to use (Magic Weapon, Magic Armor), or altering them in more direct, often superficial ways (stoneskin, invisibility). Clerics and druids, on the other hand, tend to buff in a more holistic way, changing or enhancing the basic abilities of the target, affecting them from the inside out, rather than the outside in, which follows the theme of healing and caring for people's souls.