it used to be that everything was explained in fine detail (unfortunately smaller print, too) - so library books can be excellent , actually. take for instance all the time fux spent on writing 'gradus ad parnassum.' but, then - hardly anyone has the time to go through that until several years of just buzzing through some books such as bastien's theory books or alfred's. or whatever is cheapest. i go into the music store and look for quality AND price.
or, you can order from pepper music some hardcore (fact only) books like 'master theory' by charles peters and paul yoder. they have different levels - so just start with the first three books (1-3) and you'll be well on your way. these books are workbooks - so you can write in them - unlike library books.
pepper music has a site on the internet. i think it's
www.peppermusic.com*i'd still check out the library theory books first- because you could always just buy some score paper and write the answers on that.