If you have read the Teach Yourself Logic 2015 Study Guide, then you will know that I there particularly recommend as an admirably lucid and, yes, friendly introduction to first-order logic Christopher Leary’s 2000 book, A Friendly Introduction to Mathematical Logic. Very regrettably, Prentice-Hall let this excellent book go out of print (though that was no reason not to continue recommending it in the Guide, which is largely aimed at students likely to have access to a library). But that cloud has turned out to have a silver lining. The copyright having reverted to the author, he has got together with Lars Kristiansen to produce a much expanded second edition, with seventy pages more text (on computability) and over seventy pages of solutions to exercises. A copy has just landed on my desk, and it is all looking very good. Moreover, this second edition has been published through Leary’s university library. I ordered my copy via Amazon, and evidently it is printed (on demand?) by Amazon, and very inexpensively too. So this bigger and better second edition will be notably more affordable by students.
I’ll no doubt say more about this new edition in the updated 2016 version of the Guide. In the meantime, make sure your university library gets a copy or two! (ISBN-10: 1942341075)