A good way to learn asm is through books but there are not many for current architectures (especially x64, except the official Intel manuals which are quite good but also hard to read). Nevertheless, there are some on ARM which I can recommend, namely: ARM System Developer's Guide by Sloss, Symes and Wright. ARM Assembly Language by Hohl. ARM SoC Architecture by Furber.

IDA Pro is the industry standard for reverse engineering but it also is expensive (like USD $2k). There is a free version but it doesn't offer 64bit, so not really an option for modern ObjC or Intel computers. As you've mentioned ObjC chances are you work on OS X. IDA pro is not working well on OS X (the recommended way is to use the Windows version via virtualbox and not the OS X version). Still, Hopper.app is a great alternative on OS X. Not as good as IDA, but it has a Python interface, GDB support, and decompile support for ARM, Intel (and some knowledge regarding Objc). And it's only ~USD$100. [There is also a Windows version of hopper.app but it seems not yet ready to use, as I've only heard bad things about it there so far.]