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.

I haven't really taken advantage of the discount thus far outside of one book purchase, so there are some reviewer credits laying around I'd like to use before January. I'm all stocked up on references for my own work at the moment, but 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.