Here are some User Experience (UX) guides (aka playbooks) in PDF format published by Google. They appear to be hitherto unknown to the internet as none have any back links according a check with Ahrefs backlink checker (although they're all indexed by GOOG). They will be of use to SEO consultants, web / UX designers, CRO consultants etc. They are not dated but appear relatively recent. Previous similar guides from GOOG have been discovered / circulated online for the following sectors: lead generation, automotive, travel, health care, news / publishing, finance, property & ecommerce / retail.
One is marked "Proprietary + Confidential"... 🧐
Form Optimisation (59 pages):
https://services.google.com/fh/files/events/pdf_forms_ux_playbook.pdf
Landing Page Optimisation_playbook (149 pages): https://services.google.com/fh/files/events/pdf_landing_page_optimization_playbook.pdf
Progressive Web Apps: (70 pages):
https://services.google.com/fh/files/misc/pwa_ux_playbook_2020.pdf