A personal library is a valuable tool. Below are some resources for building a good foundation for your library at a reasonable cost.
You'll know soon enough what specialty books you need--book reviews and bibliographies will help you find them.
The ones listed below will provide a broad foundation that you will find very useful

Gardner's Art Through the Ages has a fine bibliography in its last pages. This organized by period, and is a reliable guide to the "standard" books in the field--tho there are no annotations describing the sources. Many are available second hand through Amazon or other dealers at a reasonable price.

Series of Art History (and related) books. The series and publishers' offerings listed below are well-known, solid, inexpensive, and easily available second hand.

Check Amazon or other dealers for second hand offerings.

Cambridge Illustrated History series. Helpful in providing context.

Cahill, T. Hinges of History series. Cahill: "We normally think of history as one catstrophe after another, war followed by war, outrage by outrage .... But history is also the narratives of grace, the recountings of those blessed and inexplicable moments when someone did something for someone else, saved a life, bestowed a gift, gave something beyond what was required by circumstance ...."

Getty Publications series
.....Art Through the Centuries
.....Guide to Imagery
.....Looking at ...
.....Medieval Imagination

.....Getty Publications Virtual Library free art e-books online

* Medieval Craftsmen series - University of Toronto Press
.....Brief, accessible, well illustrated, written by well-known scholars

 

* New entry

Oxford University Press
.....Oxford History of Art
.....Oxford Illustrated History series Helpful in providing context.
.....Very Short Introduction series Arts & Humanities
..........Very Short Introduction - Arts

....Phaidon
..........Arts & Ideas
..........Contemporary Artists
..........Themes and Movements

....Taschen
..........Basic Art Series
..........Taschen World Architecture series (out of print, but useful)

....Thames & Hudson
..........World of Art

.....Time-Life World of Art ........

This is an older series edited by H. W. Janson, and published in the 1960s and 1970s. Each volume revolves around a major artist. It's very accessible, well illustrated according to the standards of its day, and now available for as little as a penny plus shipping per volume. Sometimes available as a complete set.

.....And for your pleasure . . .
..........AbeBook's Weird Book Room
.......* Petroski, H. The Book on the Bookshelf A delightful history
.............. of the book, and the shelves that we put our books on

Banner: Detail of David Iliff, photographer,
The Long Room of the Old Library at Trinity College Dublin, 2015.
Back / Home button is a detail of: The Town Mosaic. faience plaque,... 1700-1600 BCE. Photo by Zde
© Jan Mainzer