Skyrim's been out for four years, and I'm surprised nobody had bothered to add the simple modification of permanent bound weapons. 

The four tomes, which are basically the original spells but with unlimited timers (they still disappear upon sheathing) are with their FormID and that of its designated spell (for easy obtaining via console) as follows,

Boundless Sword
FormID: xx0012C6 (tome), xx000D62 (spell)

Boundless Battleaxe
FormID: xx00182E (tome), xx00182B (spell)

Boundless Bow
FormID: xx001831 (tome), xx001830 (spell)

Boundless Dagger
FormID: xx0046FC (tome), xx0046FE (spell)

"xx" being whatever this mod is in your load order. (for example, mine is 3B) NMM should tell you in the load order list.

For sake of convenience, the console commands you would use to easily obtain the spell are "player.additem (FormID of tome) (quantity)" and "player.addspell (FormID of spell)"

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HOW TO GET

Phinis Gestor of the College of Winterhold sells these for a fair chunk of gold along with the other original master-level spells (Flame Thrall..etc). Which means you have to have completed the Conjuration Ritual Spell!
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Because permanently bound daggers are included, this does require the Dragonborn DLC.

Please tell me if there's something wrong with this mod. ^-^

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Alternatively, if you were only looking to buff the duration of the vanilla spells, you should know there's a mod elsewhere that already exists for that- https://steamcommunity.com/sharedfiles/filedetails/?id=82252707