It is very good to note that the excellent A Friendly Introduction to Mathematical Logic by Christopher Leary and Lars Kristainsen is now available as PDF for free download from the book’s webpage here (as well as still being available as a very reasonable paperback).
Looking ahead a few weeks, I am not planning to do a serious in-depth revision of the Teach Yourself Logic Study Guide for 2018, as I really, really, must concentrate on the second edition of my intro book. But I may well do a very modest ‘maintenance upgrade’, in particular updating links where other books have become freely (but legally!) available.
Occasionally people try to post comments here noting that this or that book is electronically available at the-file-repository-of-which-we-shall-not-speak, comments which I have to delete. But I am always pleased to hear about cases like this where authors listed in the Guide have themselves made their work free-to-download: so are there any more cases I should know about?