Excellent NASEM study (also has refs for futher reading): https://www.nap.edu/catalog/25120/securing-the-vote-protecting-american-democracy

On Software Independence: https://people.csail.mit.edu/rivest/RivestWack-OnTheNotionOfSoftwareIndependenceInVotingSystems.pdf

Gentle intro to Risk Limiting Audits: https://www.stat.berkeley.edu/~stark/Preprints/gentle12.pdf

My own thoughts: https://www.mattblaze.org/papers/blaze-govtreform-20171129.pdf