best ENM book for a partner who is vanilla but curious — not too academic not too "spicy date night"

asking for a mate honestly, not me. shes early forties, husband in his mid forties, married a long time, they've been having the "what if we opened things a bit" conversation on and off for a yr now. shes the one whos curious. hes the one whos open in theory but reads a book a yr and needs to be met where he is. she asked me for a rec because she knows we're in the lifestyle and she wants something he'll actually finish. opening up by tristan taormino is on my list to suggest but i think it might be a bit much as a first read for someone whos still on the fence. and the more recent stuff like polysecure is great but i think hes not poly-curious, hes more swing-curious which is different. is there a book — ideally something that doesnt scream "swinger book" on the cover so he can read it on a plane without panicking — that genuinely works as a gentle entry point for the husband who is willing to read one (1) book before they decide anything