Which book to start with
If you're set on Spain, start with Moving to Spain: The Honest Guide. It walks through the Non-Lucrative Visa file document by document, prices three real budgets for Valencia, Málaga and Madrid, and covers healthcare after sixty and the tax rules that catch Americans.
If you haven't picked a country, start with How to Retire Abroad on $2,000 a Month. It compares what life costs in named cities across 14 countries, with health insurance priced at your actual age and a one-page worksheet to build your own number.
If you're already planning the move, Why They Came Back is about what breaks a move abroad and when, and The Restart is about rebuilding a routine, a circle and a sense of purpose after 40.
The books and this site
The site answers one question per page, with the sources linked. The books put those answers in order, from the decision to the first year after landing. Free checklists and spreadsheets from the books are on the PDFs page, and the country guides are under Countries. All books are in Kindle and paperback on Amazon.