I received an email from zbMATH today to the effect that, as a result of their going open access in January 2021 (which is awesome), they will no longer be able to offer their reviewer discount for Springer books since their distribution contract with Springer is being terminated. They advise that all reviewers use any remaining reviewer credits before January 2021, when the credits will expire.
Edit: As David White suggests in his answer below, some community members may find it preferable to donate their extra credits to mathematicians and departments in places where there is a need for books. One possible way to do this is provided by zbMATH and outlined at the bottom of their guide for reviewers as pointed out by Najib Idrissi in the comments on David's answer, allowing reviewers to donate spare credits to the EMS Committee for Developing Countries, however I have been unable to follow the link provided in the guide to see how promptly they use these donated credits (it won't load for me).
Although I've decided to follow Davids suggestion, I'm leaving up the rest of the question since others have contributed to it.
I thought it would be valuable to collect together some community recommendations to spend reviewer credits on before January 2021 since many community members here are likely reviewers for zbMATH as well.
What Springer book(s) would you recommend spending spare reviewer credits on?
I'm a fan of mathematics in general so any recommendations are welcome, but if I had to choose specific topics I'd love a good reference book on algebraic geometry, homotopy theory, combinatorics or synthetic differential geometry.
Second edit: Since the community decided to close the question, I'm going to leave it up and closed until January then delete it; there's no evident way to make the topic less subjective, and it will be irrelevant after the credits are expired. Until then, this post can still hopefully serve as a resource for people who want to donate or spend their expiring credits.