Solution to 21A has been deliberately left unsolved and is to be answered only by a non-regular / novice commenter, with proper annotation. Those who have answered earlier in the week, please give others a chance.
ACROSS
1 Finally almost got electronic (second) map books (7) ATLASES{AT LASt}{E}{S}
5 World body's back supporting companies at Thailand offering palm, say (7) COCONUT {UN}<=>{CO}{CO} and {T}
9 Taping call going over Spain's bond (9) RECORDING {R{E}{CORD}ING}
10 View of fireplace remains same if one is one (5) ANGLE (-i+a)ANGLE
11 Town house given to company (7) VILLAGE {VILLA}{GE}
12 Crooked carpenters ran off with relation (7) RESPECT {CarPEnTERS}*
13 Look CSK somehow engages (5) LOCKS {LO}{CSK*}
15 Reported it's cloudy in our country with temperature ebbing, primarily down (9) INDICATED {INDI{C}A}{Te...e}{Eb...g}{D}
17 Network usage goes to dogs adopting Gujarati primarily and dialects (9) LANGUAGES {LAN}{GUA{Gu...i}ES*}
19 Condition of jockey... (5) RIDER [DD]
21 ...say, horses is painful over a month at Sweden (7) A?I?A?S (Addendum - ANIMALS {AN{1}{M}{AL}{S} - See comments)
23 Sleeps at store, tiresome holding on (7) RETIRES [T]
25 Old artwork is bonus (5) EXTRA {EX}{ART*}
26 Identifying LA engineer and students in group (9) LABELLING {LA}{BE}{LL}{IN}{G}
27 Commercial post small news section in El Salvador? It's a pain (7) SADNESS {S}{AD}{N}{E{S}S}
28 Could be infinit (7) ENDLESS Semi&lit Definition by example
DOWN
1 A Republican challenger is coming... (7) ARRIVAL {A}{R}{RIVAL}
2 ...LA liberal overcomes officer in charge with resident (5) LOCAL {LA}{L} over {OC}
3 Shouts to get a small paste with sulphur (7) SCREAMS {S}{CREAM}{S}
4 At the end of rains, association group was trembling (9) SHIVERING {r..nS}{HIVE}{RING}
5 Racing car crashed, narc killed in smoke (5) CIGAR {racInG+CAR}*
6 Brilliant lesson. I see (7) CLASSIC {CLASS}{I}{C}
7 Unappreciated North chose to allow entry for German (9) NEGLECTED {N}{E{G}LECTED}
8 Dealt with Thailand rot with Communists outside (7) TREATED {T}{RE{ATE}D}
14 Involved criminal's sibling to exchange dinar for rupees (9) CONSISTED {CON}{SISTE(-r+d)D}
16 Advantageous for, say, NRI crowd to donate billions (9) DESIRABLE {DESI}{RABbLE}
17 Lucky engineers and doctors enjoy rare success from the beginning as pioneers (7) LEADERS Acrostic
18 Uninformed Anu returned article (7) UNAWARE {ANU<=}{WARE}
19 Disturbed that Mouse Story didn't have a decent opening (7) RATTLED {RAT}{TaLE}{De...t}
20 On indication, at the beginning of slump, quits (7) RESIGNS {RE}{SIGN}{Sl..p}
22 Special make southern fabrics (5) SILKS {S}{ILK}{S}
24 Right lane left out in rally (5) RAISE {R}{AISlE}
Reference List
Electronic = E, Second = S, Company = CO, Thailand = T, Spain = E, Company = GE, Look = LO, Cloudy = C, Down = D, Engineer = BE, Student = L, Commercial = AD, Month ,= M, Group = G, Small = S, News = N, El Salvador = ES, Republican = R, Liberal = L, Officer in Charge = OC, North = N, German = G, Thailand = T, Rot = ATE, Criminal = CON, Dinar = D, Rupee = R, Billion = B, Special = S, Southern = S, Right = R, Left = L