primer / primmer are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
primer: (noun) an introductory textbook.
primmer: NA
primer: p·r·AY·m·er
primmer: p·r·IH·m·er
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)