Grammatically, いいかなあ= いい+かな. かな(often pronounced なあ、なー、なぁ) is an ending particle that adds emotion of question. As other responses are pointing, if the speaker is talking to the listener, there is a nuance of restraint and hesitation to ask. The speaker is expecting someone would answer, but trying not to sound asking directly. It's also used without listener like Ryota-san says. かな itself is made from ending particle か(question) and な(wonder).

かなあ has also a nuance of talking oneself. such as "I'm wondering that... which book...".

どの 本 が いい かなあ? Which book will do? (かなあ adds a tone of hesitation to this sentence)

" なあ" is added to the end of the sentence, it is only an expression, it has no special meaning. どの 本 が いい かなあ? ( Which book is good ! ) An expression, question to yourself.