Books in English about Bulgarian armed forces in WW2:
Armor
Kaloyan Matev, Equipment and Armor in the Bulgarian Army.Vol. 1 ARMORED VEHICLES 1935 - 1945, Published by Angela, Sofia 2000, ISBN 954-90587-7-8, 127 pages, soft cover. size 24/16 sm. parallel text is in English and Bulgarian. This book is an illustrated (16 color illustrations and 85 black & white pictures) - the best and only study on the subject. Unfortunately out of print. The author plans to publish a new revised edition in the near future.
Aviation
Dimitar Nedyalkov, Peter Skipp
AIR POWER OF THE KINGDOM BULGARIA vol.3 1920-1939
AIR POWER OF THE KINGDOM BULGARIA vol.4 1940-1946
2001.Published by Fark Ood, Sofia, 210x280, ISBN: 9549669793, richly illustrated by photos, maps, tables, schemes, parallel text in English and Bulgarian. 60 pages each.
See:
http://rakovski-defcol.mod.bg/EN/Activi ... b_0504.php
Dimitar Nedyalkov
BULGARIAN FIGHTERS Part 2
Published by 'Propeller' Publishing, Sofia, 2004, ISBN: 954 9669 96 3. 76 pages. richly illustrated by photos, maps, tables, schemes, parallel text in English and Bulgarian.
See:
http://rakovski-defcol.mod.bg/EN/Activi ... kovBF2.php
Decorations
Todor Petrov
BULGARIAN ORDERS AND MEDALS - REFERENCE GUIDE IN ENGLISH
Published in 2005 by Voenno Izdatelstvo. 325 pages, hard cover with dust jacket, Large size 30 x 23 cm. - Luxury edition by the Bulgarian military publishing house, excellent illustrations. See:
http://bulgariana.com/product_info.php? ... 0398400c4b
Petko Pavlov
BULGARIAN ORDERS AND MEDALS
Two editions available. A must have. See:
http://www.bg-ordersandmedals.com/eng/books/pavlov.htm
Navy
Todorov Iliya
SHIPS OF THE BULGARIAN NAVY 1879-2002
Pubished by Air Group 2000, Sofia, 2003, ISBN: 9547520539, 16.50x23.50, 224 pages, B&W illustrations, text in Bulgarian and short summaries in English - the best study in both languages, unfortunately the book is out of print and the quality of the illustrations is not the best.
See:
http://www.airgroup2000.com/eng/index.p ... _kom&id=10
Choni Chonev, Peter Skipp
THE BULGARIAN NAVY - Album book Year of Publishing: 2005 Issued by: Propeller Publishing House ISBN: 954-9367-21-5 Parallel texts in Bulgarian and English languages - rather limited edition, very good quality of illustrations, some mistakes throughout the text. Still available to buy online:
http://bulgariana.com/product_info.php? ... 0398400c4b
http://www.shopbulgaria.com/index.php?m ... &id=35&s=2
OOB
George F, Nafziger
BULGARIAN ORDER OF BATTLE IN WORLD WAR II
Published by the Nafziger Collection Inc., Ohio, 1995, ISBN 1-58454-020-0, Size: 9" x 11", No illustrations, 75 pages - An Organisational History of the Bulgarian Army 1939-1945 compiled on the basis on American army intelligence data from the WW2. The overall picture is correct, but the devil is in the details, where there are numerous mistakes and omissions. Nevertheless there is no other study on the subject so far.
See:
http://home.fuse.net/nafziger/WW2_ARMY.HTM
Vehicles:
Kaloyan Matev, Equipment and Armor in the Bulgarian Army Vol.2 "TRUCKS, TRACTORS AND AUTOMOBILES 1935 - 1945" ISBN 954-90587-8-6 , Published by Angela, Sofia, November 2001. 128 pages, soft cover, size: 24 x 16 cm, parallel text in English and Bulgarian. Another good eichly illustrated study, although not as comprehensive as the one on the armor. Nevertheless a must have. It is still available for orders online.
See:
http://bulgariana.com/product_info.php? ... 0398400c4b
P.S. If I find another book in English I'll put it above. So far the list is not impressive

I'm not mentioning any Osprey titles, because so far there is not a single book from any of their series dedicated solely to Bulgaria and the sparse references found throughout their books (e.g. Axis Forces in Yugoslavia) about Bulgaria are at best dubious. I believe they simply think the topic is not interesting enough to be sold on the market...