Vectors licencing for a book that will be sold

1) Yes, a standard licence should be OK, as the assets are not the main incentive for buyers to acquire the book and your print run (combined physical and electronic) is less than 500k.

2) That's up to you to decide. As you are doing client work, and the client won't have a use for the assets above what you are doing, you can take the licence and use it for this client. If you need the same asset for a different client or yourself, you would need to take a new licence.

If the customer want's to further use the assets, you could license them for your customer, or the customer licences them and gives the assets to you for the specific client work. Both ways are possible.

If you license and give them to the customer, they need to agree on a licence agreement that is at least as stringent as that of Adobe. The client can give you in writing that they are bound by the licence terms as specified by Adobe. But you are free to write your licence terms, where the customer has to make 3 jumps and adhere to the Adobe terms if they want to use the asset. There are no required Adobe terms in this regard, except that the Adobe terms need to be respected.

Licensing is both, easy and complex at the same moment.

Look here for more information on licensing: https://community.adobe.com/t5/stock/links-for-licensing-terms/td-p/11366788
(Disclaimer: As always with licensing, this is my interpretation of the rules. I think they are correct and advice is based on reading and interpreting the licence terms and on fair use for both the buyer and the artist/stock company, but I cannot rule out that my interpretation is wrong. I'm not an Adobe employee).