Which countries? There was quite a lot of Reformation.

You really just need to search for the names of the reformers in question and check out the translations of their books available to you, see which you find most readable or which are in your library.

N.B. These are not a full lists of works by each individual concerned:
https://sourcebooks.fordham.edu/sbook1y.asp
https://eudocs.lib.byu.edu/index.php/Renaissance,_Reformation_and_Early_Modern_Germany
of the major reformers, Zwingli is missing from there:
http://oll.libertyfund.org/titles/zwingli-selected-works-of-huldrich-zwingli
Better and more recent translations of these may be available in books.

Checking: You do mean primary sources, don't you? Books written at the time and first-hand accounts, not recent scholarly texts and general non-fiction (secondary sources)?

Personally, if I had to read one primary source author from the Reformation period, it would be Erasmus because he was rather less militant than the rest of them by the standards of the time and seemed a bit nicer and more amusing. YMMV.

Sources which are not all about religion, but which allude to it, may also be of interest, for example the Chronicle of Henry Machyn: https://quod.lib.umich.edu/m/machyn/, or diaries and letters, if any survive from your country and period of interest.