Alright, here's how I look at reading levels for guide post book readers — think of it like tuning a radio to the right frequency for someone's brain. If by 'guide post' you mean books or materials meant to guide readers (or the 'Guideposts' series/magazine), the reading level can swing a lot depending on the target audience. For grown-up inspirational pieces like those in 'Guideposts', the language usually sits around a comfortable adult level: think late middle-school to high-school reading comprehension (roughly grades 8–12, or Lexile ranges from about 800L to 1100L). That makes them accessible to many adults while still offering nuance and longer sentences.
If you're dealing with actual leveled readers used in schools (guided reading, 'guidepost' markers in classrooms), then the common measures are Guided Reading Levels (A–Z), Fountas & Pinnell, DRA, and Lexile. Early emergent books (A–D) map to pre-K–1st grade, emergent (E–J) to K–2, early fluent (K–P) to grades 1–3, and transitional to fluent (Q–Z) to grades 3–6 and up. Content maturity matters, too: picture-heavy, repetitive text is for younger readers; chaptered narratives with complex themes are older-reader territory.
Practically, I pick three things when matching a reader: vocabulary density (how many unfamiliar words), sentence and paragraph length, and theme complexity. Use the independent/instructional/frustration framework — a book that a reader can read independently without stumbling more than 1 in 20 words is a keeper. If you want tools, publishers sometimes list Lexile or grade ranges; if not, paste a paragraph into a free readability checker or compare against known titles. Honestly, pairing interest with challenge is the trick — a slightly harder book that excites the reader will do more than a perfect-level book that bores them.