Respectable books on apologetics?

[Sejanus]

I don't think the words "respectable" and "apologetics" should ever be used in the same sentence.
That said, welcome to the forums. I hope you stay. You should peruse the rules, if you intend to stay here long.
As for your question, I have to admit, I haven't read many books on atheism, and certainly not any on christian apologetics. There are, however, a lot of youtube atheist channels I watch that might test your faith. Here's just a few:

Darkmatter2525 - he uses cartoons to depict his criticism of religion, which are very witty and often have a subtlety about them that makes you think.

The Amazing Atheist - he can be seen as rude, but I think he usually makes sound arguments. I would recommend his older videos, however, as his newer ones are often not as philosophical.

Thunderf00t - he's a scientist (I think he's either a nuclear physicist or a chemical engineer), and as you might expect, an intelligent guy.

TheCultOfDusty - not my favourite atheist youtuber, but he does have good arguments.

Perhaps something else you could consider would be increasing your scientific knowledge. Often a misunderstanding of the science behind, say, evolution causes problems for theists on this board. For that you should read maybe some of Dawkins' books on evolution, The Selfish Gene, for example. A Brief History of Time by Stephen Hawking would be good for a n overview of the scientific understanding of the universe.