Plans and purchase

Your title is "plans and purchase", so also some information on those:

There are annul plans and monthly plans. With an annual plan, you will need to keep the plan for a year or if you cancel early, pay an early termination fee. A monthly plan can be cancelled for the end of each month. Both plan types are paid on the month! Your month starts and ends on the day, hour, minute (maybe even second) yo take the plan. So it's safe to cancel at latest a day before renewal!

There are also credit packs, witch would probably be a good alternative for you. Credit packs expire after a year and there is no automatic renewal. With credit packs you can also buy extended licences, videos and premium images, which would all be extra or extra depending on the plan you have. 

Standard assets all cost one credit (so easy calculation: 70 assets, 70 credits) regardless of the plan you take. other assets and licence types may cost more credits or come at an extra payment with or without a discount. 

Licences never expire, so if you licence an asset now, you will be allowed to use it, even if you do not have a plan anymore. For sure, as soon as you pass the 500k limit, the standard licence is used up, so you will need to seek a solution at that moment, but up to then, they are yours to be used acconding to the licensing terms.

Sure. 

Here is why:

• The main incentive ro buy your book is the content you write.
• the print run is less than 500k.

All the rest of your indications is meaningless. I would, however, guess that you need to credit the pictures.

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).