For wall mounting bookshelves try small tv mounts made to handle there weight they are flexible and usually can give you better angles then traditional speaker mounts. The catch is you gotta Be Okay with screwing into the speaker itself to hold them and make sure you don't go through the wood ! Lol
Something like this stuff
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Just find the right size the right type to hit the angle you need you can find good mounts for as cheap as 20 to 30 bucks that can hold up to 30 pounds easy
Cheaper mounts if your speakers aren't that heavy